A critical review of all-cellulose composites
Tim Huber, Jörg Müssig, Owen Curnow, Shusheng Pang, Simon Bickerton & Mark P. Staiger Journal of Materials Science Full Set - Includes `Journal of Materials Science Letters' ISSN 0022-2461 Volume 47 Number 3 J Mater Sci (2012) 47:1171-1186 DOI 10.1007/s10853-011-5774-3
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Author's personal copy J Mater Sci (2012) 47:1171–1186 DOI 10.1007/s10853-011-5774-3
MATERIALS IN NEW ZEALAND
A critical review of all-cellulose composites Tim Huber • Jo¨rg Mu¨ssig • Owen Curnow Shusheng Pang • Simon Bickerton • Mark P. Staiger
•
Received: 9 February 2011 / Accepted: 7 July 2011 / Published online: 21 July 2011 Ó Springer Science+Business Media, LLC 2011
Abstract Cellulose is a fascinating biopolymer of almost inexhaustible quantity. While being a lightweight material, it shows outstanding values of strength and stiffness when present in its native form. Unsurprisingly, cellulose fibre has been rigorously investigated as a reinforcing component in biocomposites. In recent years, however, a new class of monocomponent composites based on cellulosic materials, so-called all-cellulose composites (ACCs) have emerged. These new materials promise to overcome the critical problem of fibre–matrix adhesion in biocomposites by using chemically similar or identical cellulosic materials for both matrix and reinforcement. A number of papers scattered throughout the polymer, composites and biomolecular science literature have been published describing non-derivatized and derivatized ACCs. Exceptional mechanical properties of ACCs have been reported that easily exceed those of traditional biocomposites. Several
T. Huber M. P. Staiger (&) Department of Mechanical Engineering, University of Canterbury, Private Bag 4800, Christchurch, New Zealand e-mail:
[email protected] J. Mu¨ssig Department for Biomimetics, University of Applied Sciences Bremen, Neustadtswall 30, 28199 Bremen, Germany O. Curnow Department of Chemistry, University of Canterbury, Private Bag 4800, Christchurch, New Zealand S. Pang Department of Chemical and Process Engineering, University of Canterbury, Private Bag 4800, Christchurch, New Zealand S. Bickerton Department of Mechanical Engineering, University of Auckland, Tamaki Campus, Auckland, New Zealand
different processing routes have been applied to the manufacture of ACCs using a broad range of different solvent systems and raw materials. This article aims to provide a comprehensive review of the background chemistry and various cellulosic sources investigated, various synthesis routes, phase transformations of the cellulose, and mechanical, viscoelastic and optical properties of ACCs. The current difficulties and challenges of ACCs are clearly outlined, pointing the way forward for further exploration of this interesting subcategory of biocomposites.
Cellulose—the natural choice for composite materials Introduction Cellulose is one of the most abundant biopolymers on earth with *1.5 9 1012 tons of cellulose produced each year. Thus, it presents an enormous amount of a renewable and biodegradable resource for raw materials [1, 2]. Cellulose fibres are widely recognised for their applicability in ecofriendly composite materials, although unlocking their full potential remains a challenge for load-bearing engineering applications. Chemistry and phases of cellulose The molecular composition of cellulose, isolated from plant cell walls, was first discovered and determined by Anselme Payen (1795–1871). Next to plants, some algae, fungi and bacteria species are also produce cellulose [1]. Cellulose is a linear polymer composed from aldehyde sugars, so-called D-anhydroglucopyranose units (C6H11O5/ IUPAC nomenclature: (3R,4S,5S,6R)-6-(hydroxymethyl) oxane-2,3,4,5-tetrol), often simply referred to as glucose
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Fig. 1 Molecular structure of a cellobiose unit
units, assembled into groups of two as ‘‘cellobiose’’ units. Figure 1 shows the typical molecular structure of a cellobiose unit. A single glucose unit is a hexose that takes on one of two forms (a or b), depending on the position of the hydroxyl groups. Individual cellulose chains are highly hydrophilic due to the large numbers of hydroxyl groups present. Native cellulose or cellulose I is the most crystalline type of which there are two forms: Ia and Ib. While the cellulose Ia crystal has a triclinic unit cell, the cellulose Ib crystal has a monoclinic unit cell. Both, cellulose Ia and Ib are present in native cellulose structures but their ratio depends on the source of cellulose. Other allomorphs of cellulose are possible out of which the most common are cellulose II, III, and IV. Cellulose II can be formed by mercerisation or regeneration of cellulose I [1, 3–5]. Cellulose III can be formed from either, cellulose I or cellulose II by a treatment with liquid ammonia, resulting in either cellulose III1 or cellulose III2. Cellulose IV1 and Cellulose IV2 can be prepared by the corresponding form of cellulose III by heating in glycerol [6]. Single-molecule cellulose chains interconnect via hydrogen bonds to form cellulose microfibrils that exhibit crystalline, paracrystalline and amorphous regions [7]. Those microfibrils are present in the secondary cell wall of all plants, usually embedded in a matrix consisting of hemicelluloses and lignin. The degree of polymerisation (DP) of cellulose varies widely depending on the source, ranging from 300 in wood fibres up to 10,000 for plant fibres and bacterial cellulose. Cellulose content, DP and the lateral arrangement of the microfibrils determine the tensile properties of a plant fibre. Cellulose microfibrils can be classified as nanomaterials given the lateral dimensions of a microfibril is in the range of 5–50 nm [1, 8–10]. Mechanical properties of cellulose An average Young’s modulus of 10.3 GPa was calculated for amorphous cellulose using a force-field model [11]. Using X-ray diffraction, Sakurada et al. [12] determined the Young’s modulus of elementary cellulose fibril of bleached ramie fibre to be 134 GPa. Using a similar set-up, Nishino et al. [13] measured the elastic modulus of several cellulose polymorphs. Cellulose I was found to have a modulus of 138 GPa, whereas cellulose IV exhibited a
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lower value of 75 GPa. Ishikawa et al. [8] published a similar ranking for those polymorphs sourced from ramie fibre. This variation in tensile properties is due to differences in the molecular structure of the different allomorphs. Deformation mechanisms involve complex stretching and re-organisation of the hydrogen bonds, which differs greatly for crystalline and amorphous phases. The mechanical properties of cellulose compete well with other engineering materials such as aluminium (70 GPa) or glass fibres (76 GPa) [14]. As a result of the low density of 1.58–1.59 g/cm3 Wegst et al. ranked the specific stiffness of native cellulose of 67 GPa cm3 g-1 as among the highest of all natural materials [5, 15, 16]. Cellulose I is the strongest allomorph with a theoretical ultimate tensile strength of about 13–17 GPa. Cellulose II and amorphous cellulose are less strong with tensile strengths of *9 and 0.8 ± 0.1 GPa, respectively [17]. The high tensile strength and low density of the native cellulose crystal results in the highest specific tensile strength of any known natural polymers for cellulose I (667 MPa cm3 g-1) [15]. The biocomposite development Early reports on the use of natural fibres in composites date back to the early 1970 and 1980 [18, 19], since then modern advances in the development of cellulose fibrereinforced polymer composites have been the subject of several hundred studies. Due to the independence of cellulosic fibres of crude oil and their vast availability, an improved CO2-balance compared with composites made from industrially made fibres and fillers and good mechanical properties, cellulose-containing composites have generated much interest amongst various industries, especially the automotive industry [20–28]. The most commonly used natural fibres for composite applications are wood, jute, flax, sisal and hemp, although many others are also suitable for biocomposites [29–33]. The high specific tensile strength and stiffness of natural fibres makes them a lightweight alternative to traditional reinforcements such as glass fibres or other fillers. Natural fibres are also less hazardous to handle and require less energy during processing compared with glass or carbon fibres. The fibres themselves also sequestrate carbon dioxide during growing and are biodegradable [34]. However, due to different growing conditions, natural fibres usually show a large scatter of properties compared to industrially made glass fibres. A strong quality management during fibre harvesting and processing and extreme care while determining fibre properties are necessary to produce reliable and reproducible results [35, 36]. Several traditional processing methods for thermosetting and thermoplastic polymers have been modified to allow the use of
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natural fibres. New processing routes have also been developed that allow more rapid fabrication of biocomposite components at production rates demanded by industry [37]. A promising development in biocomposites research has been the transition from petroleum-based polymer (e.g. polyethylene, polypropylene) to naturally derived biopolymer (e.g. polylactides, palm oil-based resins, starch) matrices to produce composites that aim to be completely biodegradable and CO2 neutral [32]. While composites based on cellulose reinforcement and petroleum-based polymers are usually referred to as eco- or biocomposites, composites based entirely on naturally derived fibres and biopolymers have been named green (bio) composites [32, 34]. Bio- and green composites are finding applications in a wide range of applications from structural to biomedical [32, 38–42]. Nevertheless, the inherent chemical incompatibility between a hydrophobic polymer matrix and hydrophilic cellulose [43, 44] causes interfacial bonding between the cellulosic and biopolymer components to be often weak, particularly in the case of thermoplastic biopolymers [45]. This leads to an inefficient stress transfer under load and thus low mechanical strength and stiffness [20, 43, 46, 47]. The chemical compatibility can be improved by a chemical treatment of the fibre or matrix. Silane, alkaline, acetylation, chemical grafting, and corona discharge treatments provide widely varying degrees of improvement [29, 45, 48–53]. Interfacial bonding can also be increased by using nanosized forms of cellulose such as bacterial cellulose [54, 55], microfibrillated cellulose [56, 57] and cellulose whiskers [58–61] that provide an increased surface area per volume. While significant improvements in mechanical properties can be obtained, the above methods also add cost and complexity to the formulation of biocomposites. Cellulose reinforced cellulosic structures such as vulcanized cellulosic fibres have been reported decades ago and found applications, for example, as vulcanized paper [62]. However, growing environmental awareness and increasing interest in sustainable material concepts have lead to the development of bio- and green composites for structural composite applications. The newly developed all-cellulose composites (ACCs) described in this review represent an approach to formulating green composites that aim to eliminate the chemical incompatibilities between reinforcement and matrix phases by utilising cellulose for both components. ACCs show the potential to be the next step in the development of more sustainable composites. The processing, characterisation, properties and applications of this promising class of high strength biocomposite materials is presented in detail in the first section of this review.
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Other approaches to produce all-cellulosic composites involve chemical treatments, such as oxypropylation or benzylation, to generate cellulose derivatives that form the matrix phase and are covered in the second part of this review.
Non-derivatized all-cellulose composites Introduction The commonly accepted definition of a composite is a material that consists of two or more distinct materials to improve the stiffness, strength and/or toughness over the individual constituents. However, in a monocomponent or single polymer composite, reinforcing and matrix phases are based on the same material. Theoretically, this would lead to an interfaceless composite where boundaries between reinforcement and matrix are indistinct in the presence of ideal chemical bonding. Therefore, the need for energy intensive fibre treatments or coupling agents for improving interfacial bonding could be drastically reduced or even completely eliminated. While the reinforcement and matrix of monocomponent composites are necessarily of the same chemical composition, physical morphology and/or structural phases of the two components may differ in reality. The performance of a monocomponent or single-polymer composite is best illustrated with an example of the concept as put forward by Capiati and Porter [63]. In this work, high-density polyethylene composites were produced with a gradient of changing morphology between the reinforcing fibres and the matrix material resulting in an improved interfacial shear strength in the range between glass fibre-reinforced polyester and epoxy resins. In addition to the enhanced bonding at the reinforcement–matrix interface, monocomponent composites can also provide a more straightforward path for recycling as the fibre and matrix do not require separating (e.g. all-polypropylene composites [64–67]). A recent summary of different single polymer composites is presented by Matabola et al. [68]. The concept of an all-cellulose composite was first discussed by Nishino et al. [14]. ACCs can be considered bio-derived monocomponent composites; although strictly speaking, the same source of cellulosic materials would need to be used for the reinforcing and matrix phases. While the ease of recycling is an important advantage for thermoplastic-based monocomponent composites, the main driver for the development of ACCs is to improve chemical bonding at the reinforcement–matrix interface. During the processing of ACCs, it is quite possible to have two or more different allomorphs present. Cellulose molecules strongly interact through hydrogen bonding, although the
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interaction between different cellulose allomorphs has not been quantified. Thus, the details of chemical bonding at allomorph boundaries in ACCs remain elusive. The morphological characteristics of the interface have only been observed qualitatively using for example scanning [69] or transmission electron microscopy [70]. Preparation and synthesis of ACCs Processing routes There are two distinct strategies in the literature for the preparation of ACCs (Fig. 2). The first of these methods (2-step method) involves firstly dissolving a portion of cellulose in a solvent which is then regenerated in the presence of undissolved cellulose. An example of this method was first given by Nishino et al. [69] in which Kraft fibre was fully dissolved and then regenerated in the presence of ramie fibres. A second route (1-step method) involves partial dissolution of the surface of cellulosic fibres then regenerated in situ to form a matrix around the undissolved portion. An example of this method was first given by Gindl et al. [71] in which they partially dissolved cellulose I, resulting in volume fractions of up to 90% of the original fibre and 10% of newly regenerated cellulose matrix. This method has also been described as ‘‘surface selective dissolution’’ [97]. In these processing routes, the dissolution step is followed by solvent removal and cellulose regeneration using water or other coagulants, after which the composites usually have to be dried. Cellulose dissolution Known non-derivatising solvents for cellulose include lithium chloride/N,N-dimethylacetamide (LiCl/DMAc),
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dinitrogen tetroxide/dimethylformamide (N2O4/DMF), N-methylmorpholine-N-oxide (NMMO), mineral acids, sodium hydroxide (NaOH), dimethylsulfoxide/tetrabutylammonium fluoride (DMSO/TBAF), dimethylimidazolone/lithium chloride, and various molten salt hydrates and ionic liquids (ILs). Of these, LiCl/DMAc, NMMO, NaOH and the ionic liquid 1-butyl-3-methylimidazolium chloride (BmimCl) have been used mostly in the processing of ACCs (Table 1). However, limited dissolution capacity, slow dissolution rates, toxicity and non-recyclability are the reasons that prevent some of these solvents from being used in large industrial scales. It has been observed that some ILs offer high cellulose dissolution rates. The low vapour pressure of ILs also makes them easy to reuse and safer to handle, and has led to the term ‘‘green solvents’’ [72–76]. In the following, we review in further detail properties of cellulose solvents used for the production of ACCs. NMMO belongs to the family of cyclic, aliphatic, tertiary amine oxides, where the nitrogen carries the cyclic and aliphatic groups, and oxygen [81] (compare Fig. 3). The highly polar N–O group is responsible for the high hydrophilicity of NMMO and its complete miscibility in water, as it readily forms hydrogen bonds. NMMO is a powerful cellulose solvent due to the high polarity and weakness of the N–O bond [81]. NMMO is used industrially in the Lyocell process for producing regenerated cellulose fibres. The main steps of this process are the preparation of the slurry by dissolution of cellulose (usually pulp or cotton) in a mixture of water, NMMO, stabilizers and additives. The cellulose solubility depends on the mixing ratio of cellulose, water and NMMO. A more detailed description of the dissolution process and the influencing factors can be found in the review of Fink et al. [82]. The dissolution is followed by an extrusion of the viscous dope at elevated temperatures Table 1 Cellulose solvents used for the production of all-cellulose composites and the year their functionality was reported Solvent
Year
Reference
NMMO
1969
Johnson [77]
LiCl/DMAc
1981
McCormick [78]
NaOH–urea
1995
Isogai and Antalla [79]
Ionic liquids
1934
Graenacher [80]
Fig. 3 Structural formula of the NMMO molecule
Fig. 2 Schematic of two-step (a) and one-step (b) all-cellulose composite preparation. The scheme of the one-step process is adapted from Nishino and Arimoto [95]
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(generally 90–120 °C) through an air gap. The fibres are then coagulated into a precipitation bath, washed and dried. Around 99% of the NMMO can be recovered from the precipitation and washing baths [83]. Lyocell fibres primarily consist of monoclinic cellulose type-II crystallites [82, 84]. Crystallites with a length, width and thickness of 12–14, 8–10, and 3–4 nm, respectively, accumulate into strand-shaped bundles with lengths of 150–550 nm, partly assembled into aggregates of 30–60 nm in diameter [84]. The strength and stiffness of regenerated fibres formed from NMMO in the presence of additives can be up to 1.3 and 55 GPa, respectively [85]. Another solvent for the preparation of ACCs is DMAc mixed with LiCl (see Fig. 4). Cellulose needs to undergo a so-called ‘‘activation procedure’’ during which the fibre is penetrated with a polar medium [86]. Without activation, it can take several months for the dissolution to proceed regardless of the crystallinity of the cellulose. Amorphous cellulose obtained by ball-milling also proves difficult to dissolve in the absence of the activation procedure [87]. Interestingly, the activation step before the actual dissolution does not affect cellulose crystallinity [88]. There are two different ways to prepare the mixture: (i) the LiCl/DMAc solution is prepared first and then the cellulose is added, or (ii) cellulose and DMAc are mixed together followed by addition of LiCl [89]. Stirring is also critically important for the dissolution to proceed due to the heterogeneous fibre-solvent mixture. It has been reported that the solubility of cellulose increases with LiCl content [90]. Many studies on ACCs report the use of LiCl/DMAc [14, 69, 71, 91–99], which may be due to its ability to completely dissolve high molecular weight cellulose [88]. In those studies, a concentration of 8 wt% LiCl was used for the dissolution of cellulose. A more eco-friendly non-derivatizing solvent for cellulose is based on NaOH or aqueous NaOH solutions with additions of urea and/or thiourea used at sub ambient temperatures or other additives such as poly(ethylene glycol) (PEG) or zinc oxide [100, 101]. NaOH-urea-thiourea dissolution is a simple, safe process requiring minimal energy input. The addition of urea ((NH2)2CO) and/or thiourea ((NH2)2CS) to aqueous NaOH greatly enhances Fig. 4 Structural formula of the LiCl/DMAc molecule
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the dissolution, while NaOH can only partially dissolve celluloses of low DP [102–105]. The alkaline solvent is cooled to subzero temperatures. Once the subzero temperature is attained, the mixture is often stirred under thawing, resulting in cellulose dissolution [102, 106]. The solution can be centrifuged to separate the undissolved portion from the truly dissolved cellulose [107, 108]. The dissolved cellulose is then transformed into a gel by a thermal path, precipitated in an acidic medium or coagulated. Depending on the process, different microstructures can be obtained [109]. Coagulation was used to produce cellulose membranes with varying pore geometries and mechanical properties according to the coagulant type, concentration and coagulation time [110–112]. NaOH– urea (NaOH/(NH2)2CO) and NaOH–thiourea (NaOH/ (NH2)2CS) can also be used to process the cellulose into textile fibres with mechanical properties close to commercially available rayon fibre [102, 103]. Graenacher [80] was the first to discover an IL solvent system for cellulose, but this was thought to be of little practical value at the time. Much more recently the use of ILs as a solvent for cellulose has been reported by Swatloski et al. [73]. Ionic liquids are molten salts with melting points below 100 °C. There is a wide range of possible cations (e.g. alkylimidazolium ([R1R2IM]?), tetraalkylammonium ([NR4]?) and tetraalkylphosphonium ([PR4]?) and anions (e.g. hexafluorophosphate ([PF6]-), nitrate ([NO3]-) or chloride, bromide and iodide salts [113]. Only some of them are able to dissolve cellulose, but the number of possible ion combinations is said to be as high as one trillion which leaves much scope for the development of new types of cellulose solvents [114]. Their ability to dissolve cellulose originates from their high effective polarity, due to their ionic character. The most successful ILs in cellulose dissolution reported so far, are hydrophilic and consist of the cations methylimidazoloium and methylpyridinium cores with allyl-, ethyl- or butyl side chains with chloride, acetate or formate anions [115, 116]. Recently, Pinkert et al. [117] provided a detailed review of cellulose dissolution by ionic liquids. ILs combine all of the desirable characteristics of the previous solvents including low volatility, low cost due to ease of recycling, capacity for rapid and complete dissolution of a broad range of cellulose sources and with no requirement for pre-treatment or activation. However, some ILs have proven to be toxic and an environmental hazard, contradicting their image as ‘‘green’’ solvents [118, 119]. Influence of cellulose sources on dissolution Various cellulosic materials including wood pulps, ramie, sisal and regenerated cellulose fibres, microcrystalline cellulose powder, bacterial cellulose and filter paper have been used to produce ACCs. The time required for
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cellulose dissolution depends strongly on the fibre structure, especially the degree of orientation and crystallinity in the outer part of the fibre. Thus, the dissolution conditions need to be tailored to different sources of cellulose in forming ACCs. Soykeabkaew et al. compared immersion times necessary to form a matrix phase of highly orientated cellulose structures present in Bocell fibres to less ordered cellulose configurations in the outer regions of Lyocell fibres. They reported that the Bocell fibres needed more than 1 h immersion time to form a matrix phase while for the Lyocell fibres less than 20 min were sufficient. Longer immersion times also lead to a reduction in fibre diameter as the fibre surface dissolution increases [97, 98]. Cellulose regeneration The steps involved in the regeneration of the dissolved cellulose are (i) removal of the solvent by a coagulant (water, alcohol or acetone are commonly used) and then (ii) removal of the coagulant through evaporative drying. Cellulose regeneration is an important step in processing ACCs as it controls the precipitation of the final cellulose phases. Duchemin et al. suggested that the regeneration rate controls the phase composition, which in turn will dictate the physical properties of the ACC. Cellulose phases of higher crystallinity are observed as the rate of regeneration is decreased. This is thought to be due to the dissolved cellulose chains having greater time to order themselves into a lower energy configuration. Thus, the rate of application of the coagulant for removal of the solvent and then subsequent drying rate can be manipulated to given varying properties in the final ACC [70]. Contact of the coagulant with the cellulose will lead to swelling especially if water is used. Distortion of the sample due to warping is even more apparent in thicker samples as a diffusion gradient of coagulant from the surface to the interior of the solution results in differential shrinkage and subsequent delamination and void formation. Furthermore, the dissolved and undissolved portions of cellulose will swell by different amounts which upon regeneration again can cause differential shrinkage during regeneration, leading to the formation of voids at the fibre– matrix interface [70]. Phase characterisation Identification and characterisation of cellulose phases present in ACCs has been mainly carried out with wideangle X-ray scattering (WAXS) [69, 71, 92, 95, 97, 115] and to a lesser extent solid state nuclear magnetic resonance (NMR) [120]. The treatment of cellulose with LiCl/DMAc leads to a decrease in crystallinity depending on the immersion time.
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Longer dissolution times lead to a reduction of cellulose I crystallites resulting in a change of crystallinity after regeneration of the cellulose. Nishino et al. [69, 95] and Soykeabkaew et al. [97] reported that after dissolution with LiCl/DMAc the regenerated phase was non-crystalline or amorphous based on WAXS. On the contrary, Duchemin et al. [120] obtained results from WAXS and NMR that suggest that exposure of crystalline cellulose to LiCl/ DMAc results in peeling away of thin layers from the original crystallites which retain some molecular ordering. After solvent removal, these thin layers can form a paracrystalline phase that is distinct from typical amorphous cellulose and closer in structure to cellulose I. They also suggested that the presence of a paracrystalline ‘‘matrix’’ is one of the underlying reasons for the high mechanical properties of ACCs. Zhao et al. [115] observed that dissolution of cellulose I using the IL BmimCl results in a matrix phase consisting of cellulose II. Gindl and Keckes [71] also identify the regenerated phase of dissolved cellulose I as cellulose II. Mechanical properties The mechanical properties of anisotropic composites strongly depend on the dissolution and regeneration conditions. The longitudinal tensile strength decreases with increasing dissolution time due to a decreasing crosssectional area of the load-bearing cellulose fibrils and therefore a reduction in fibre volume fraction. The transverse tensile strength follows an opposite trend as the matrix phase increased and the interface becomes more homogenous. Soykeabkaew et al. [97] reported that assumption based on their experiments with unidirectional ramie fibre composites. Immersion times of 12 h lead to a decrease in longitudinal tensile strength of about 60% compared to immersion times of 6 h. Simultaneously, the transversal tensile strength is increased by about 33%. However, there are very few publications stating transversal strength and stiffness of unidirectional composites to verify this assumption. Extending the dissolution time can lead to over-dissolution of the fibres, resulting in a rapid decrease in tensile properties for either isotropic or anisotropic ACCs [69, 99]. The application of the solvent can also affect the composition of the reinforcing fibres. Gindl et al. [93] observed that the hardness of the fibres changes when treated with LiCl/DMAc, presumably due to partial dissolution of cellulose within the cell walls. The inherent properties of the reinforcement will also affect the properties and processing of ACCs. Many different combinations of fibre, matrix, and solvent systems have been studied in the literature, giving a large range of properties for ACCs. Table 2 lists the tensile properties of ACCs made using different materials and solvents,
Cellulose source for matrix
MCC
Filter paper
18
19
Cellulose source for reinforcement
(ISO)
(ISO)
(ISO)
(ISO)
(ISO)
(UD)
(UD)
(UD)
(UD)
(ISO)
(ISO)
(ISO)
Solvent
Reinforcement type
Rice husks (ISO)
Rice husks (ISO)
Ramie fibre (UD)
Ramie fibre (UD)
Hemp fibre (ISO)
Cellulose whiskers (ISO)
Cellulose whiskers (ISO)
Cellulose source for reinforcement
Ramie fibre
22
Ramie fibre (UD)
Ramie fibre (UD)
(ISO)
LiCl/DMAc LiCl/DMAc
LiCl/DMAc
88
73
72
85
16
80
Immersing in water, acetone, DMAc Mercerization
Wet drawing to align cellulose fibrils 85
80
Fibre volume fraction (%)
124
91.8
411
105.7
58.7
910
350
250
480
211
154
242.8
Tensile strength : (MPa)
56
57.5
410
480
28.9
117
124
Tensile strength : (MPa)
540
428
400
Tensile strength :(MPa)
29
Tensile strength ? (MPa)
12
10.8
5.75
18
6.9
3.2
23
12
9
26
8.2
12.2
13.1
Young’s modulus : (GPa)
2.92
1.74
25
1.8
5.9
5.1
17 95
25 25
33.5
2.76
5.67
4.8
4
20.8
Strain to failure : (%)
2
3.76
4.3
3.3
2.5
8.2
10
24
3.7
3.8
0.023
8.6
4.8
2.3
3
Strain to failure : (%)
Strain to failure : (%)
Young’s modulus : (GPa)
Young’s modulus : (GPa)
Tensile strength ? (MPa)
Tensile strength ? (MPa)
4.5
21
Strain to failure ? (%)
Strain to failure ? (%)
5
Strain to failure ? (%)
MCC microcrystalline cellulose, BC bacterial cellulose, LDR low draw ratio, HDR high draw ratio
Included are the types of cellulose source, reinforcement, solvent and fibre fraction used. Both tensile properties parallel (:) and transverse (?) to the fibre direction are given where available
Wood pulp
MCC
20
21
60
40
85
80
40
20
10
Fibre volume fraction in %
Fibre volume fraction in %
Additional processing step
Ionic liquid (BmimCl)
Ionic liquid (BmimCl)
LiCl/DMAc
LiCl/DMAc
LiCl/DMAc
LiCl/DMAc
LiCl/DMAc
LiCl/DMAc
LiCl/DMAc
LiCl/DMAc
LiCl/DMAc
LiCl/DMAc
Solvent
Ionic liquid (BmimCl)
Ionic liquid (BmimCl)
LiCl/DMAc
LiCl/DMAc
NMMO
NaOH/urea
NaOH/urea
Solvent
All-cellulose composites prepared with further fibre or composite processing
Cellulose source for matrix
BC
17
No.
MCC
MCC
15
16
HDR-Lyocell fibre
Bocell fibre
13
14
Ramie fibre
LDR-Lyocell fibre
11
Filter paper
10
12
MCC
Beech pulp
8
9
One-step process
Cellulose source for matrix and reinforcement
Filter paper
7
No.
Ramie fibre
Filter paper
5
6
Cellulose powder
Wood pulp
3
4
Cotton linter pulps
Cotton linter pulps
1
2
Two step process
No.
Qin et al. [99]
Gindl and Keckes [92]
Nishino et al. [69]
Reference
Duchemin et al. [126]
Duchemin et al. [126]
Soykeabkaew et al. [96]
Duchemin et al. [70]
Duchemin et al. [70]
Soykeabkaew et al. [98]
Soykeabkaew et al. [98]
Soykeabkaew et al. [98]
Soykeabkaew et al. [97]
Nishino and Arimoto [95]
Gindl et al. [93]
Gindl and Keckes [71]
Reference
Zhao et al., 2009, [115]
Zhao et al., 2009, [115]
Qin et al. [99]
Nishino et al. [69]
Quajai and Shanks [122]
Qi et al. [125]
Qi et al. [125]
Reference
Table 2 Overview over isotropic (ISO) and unidirectional (UD) all-cellulose composites produced by one and two step processes and after additional fibre or composite treatment
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Fig. 5 Ranges of tensile strengths and Young’s moduli of isotropic ACCs (dashed) compared with traditional isotropic biocomposites (dotted). The numbers and letters of the references are found in Tables 2 and 3, respectively
Fig. 6 Ranges of tensile strengths and Young’s moduli of unidirectional ACCs (dashed) compared with traditional unidirectional biocomposites (dotted). The numbers and letters of the references are found in Tables 2 and 3, respectively
demonstrating the large variability possible with formulation and processing. Thus, judging and comparing the influence of various parameters and properties between different studies of ACCs is difficult. However, it is of interest to compare the family of ACCs with other biocomposites. In general, the tensile strength of ACCs is significantly higher compared with the more traditional isotropic and unidirectional biocomposites (compare Figs. 5, 6; Table 3). Interestingly, a comparison of unidirectional ACCs with traditional biocomposites does not reveal dramatic differences in the Young’s modulus. The underlying reasons may be complex given the variable formulations but may be due to either (i) the cellulose solvent decreasing the modulus of the reinforcing fibres in ACCs or (ii) the modulus of biocomposites being dominated by the modulus of the fibres, with the fibre-matrix interfacial strength being less important. In contrast, significant increases in modulus are observed for isotropic ACCs compared with traditional biocomposites in which it could be envisaged that the high matrix properties of ACCs dominate this behaviour.
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The strain to failure of ACCs (eACC ) is largely dominated f by the type of reinforcement used. For example, ACCs reinforced with low strain ramie fibre (ef = 1.2–3.8%, [121]), show lower values of eACC (3.7–4.8%) when comf pared with reinforcement made from high strain Lyocell fibre (ef = 9.4–27.9%, [98] which gives higher eACC f (10–24%) (Table 2). ACCs based on hemp fibre reported by Quajai et al. [122] achieved values of 20% for eACC . This is f unexpected as ef of hemp fibres is only 1.6% [121]. In fact, this last example emphasises the difficulty in comparing different ACCs in the literature due to differences in processing that can greatly influence the final properties. Other chemical or mechanical processing steps have been used to influence ACC properties (Table 2). The positive effect of a treatment with an alkali solution aso known as ‘‘mercerization’’ on lignocellulosic fibres, namely an improvement of tensile properties and absorption characteristics, is well known [123, 124]. When applied to the ACCs, a swelling of the fibres occurs that fills in voids and cracks significantly improving the interface and, therefore, the tensile properties of the composite. SEM pictures of an untreated (a) and mercerized (b) composite can be seen in Fig. 7 [99]. Furthermore, Gindl et al. reported that wet drawing of the composites after regeneration of the dissolved cellulose could change the orientation of the cellulose crystals within the composite towards a unidirectional direction according to the direction of the applied load. The wet state changes the molecule mobility as the water adsorption weakens the inter- and intramolecular hydrogen bonds. By drawing the composites, the crystal orientation changes linearly with the draw ratio, the ratio between specimen length after and before stretching, whereas the overall crystallinity of the composite stays unaffected. Drying the samples afterwards causes the molecule chains to keep their positions resulting in anisotropy of the specimens with an improved tensile strength in longitudinal direction [91, 92]. Viscoelastic properties The viscoelastic properties of ACCs are determined by the viscoelastic properties of the cellulose chains and allomorphs. Amorphous cellulose exhibits a viscoelastic behaviour involving different molecular motions depending on temperature [135, 136]. Due to its high content of hydroxyl groups, hydrogen bonds are formed with neighbouring units of the same molecule, with neighbouring chains and any water that is present. Intra- and intermolecular hydrogen bonds are responsible for the thermal stability of the cellulose molecule. Intramolecular hydrogen bonds increase the stiffness of the polymer while the intermolecular hydrogen bonds and van der Waals interactions are responsible for single chains arranging into a
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Table 3 Overview over the tensile properties of different isotropic and unidirectional biocomposites Matrix
Fibre type
Fibre fraction in (vol.% or wt%)
Tensile strength (MPa)
Young’s modulus (GPa)
Strain to failure (%)
Reference
Isotropic composites PBAT
Flax
30 (vol.%)
32
4.1
2
Bodros et al. [25] (Fig. 5a)
PBS PHB
Flax Flax
30 (vol.%) 30 (vol.%)
49 40
3.8 4.5
2.5 1.8
Bodros et al. [25] (Fig. 5b) Bodros et al. [25] (Fig. 5c)
PLA
MCC
5 (wt%)
31.9
1.5
[100
Petersson and Oksman [127] (Fig. 5d)
PLA
Cellulose whiskers
5 (wt%)
47
2.1
5.4
Mathew et al. [128] (Fig. 5e)
PLA
Cellulose microfibres
5 (wt%)
59
2.3
3.3
PLA
Cordenka
40 (vol.%)
57.97
4.85
–
Mathew et al. [128] (Fig. 5f) Bax and Mu¨ssig [129] (Fig. 5g)
PLA
Flax
40 (vol.%)
54.15
6.31
–
Bax and Mu¨ssig [129] (Fig. 5h)
PCL
Starch nano-crystals
50 (wt%)
15.5
0.384
5.0
Habibi and Dufresne [40] (Fig. 5i)
PCL
Cellulose nano-crystals
50 (wt%)
18.7
0.442
8.6
Habibi and Dufresne [40] (Fig. 5j) Sreekuma et al. [130] (Fig. 5k) Mu¨ssig et al. [42] (Fig. 5l)
Polyester
Banana
40 (vol.%)
68
1.87
6
PTP
Hemp
21 (wt%)
63
7
–
Unidirectional composites Epoxy
Flax
50 (wt%)
119
30
–
Bos [29] (Fig. 6m)
PP
Flax
55 (vol.%)
320.7
28.2
–
Madsen and Liholt [131] (Fig. 6n)
PP
Jute
21.2 (vol.%)
141
11
–
Khondker et al. [132] (Fig. 6o)
Epoxy
Flax
49 (vol.%)
284
26
–
van de Weyenberg et al. [133] (Fig. 6p)
PLA
Kenaf
70 (vol.%)
230
24
–
Ochi [134] (Fig. 6q)
Starch
Flax
60 (wt%)
78
9.3
–
Romhany et al. [44] (Fig. 6r)
Epoxy
Sisal
46 (vol.%)
211
19.7
1.9
Oksman et al. [33] (Fig. 6s)
Fig. 7 SEM pictures of ramie fibre-reinforced cellulose composite made with 4% cellulose concentration solution in an untreated (a) and mercerized (b) state [99]. Reprinted from Ref. [99]. Copyright 2008, with permission from Elsevier
2D sheet [137]. The stable structure of the cellulose molecule is the result of a dense network of hydrogen bonds and their different bonding patterns within a single cellulose structure and their dependency on different temperatures [138]. At low temperatures, the molecular mobility is considered as localised at the molecular level, giving rise to secondary relaxations [139]. Two secondary relaxations of amorphous cellulose are reported, occasionally referred to as ccell and bcell [140, 141]. The c-relaxation is reported to appear at a constant temperature of -123 °C. It has been
shown by the analyses of other polysaccharides such as dextran, that the c-relaxation is mainly associated with a rotation of the CH2OH groups of the cellulose molecule rather than the rotation of its OH groups [140]. The bcellrelaxation is associated with cooperative but localised motion of segments of the main chain of the cellulose molecule depending on the water content [139]. The temperature range for the bcell-relaxation was reported as -83.15 to 43.15 °C [140, 141]. At higher temperatures, cellulose exhibits three further transitions, designated a1, a2 and a3 by Manabe et al. [142] and measured for regenerated
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cellulose. The a3 transition is very closely related to the water content, and therefore not associated with the inherent molecular motion but with the cooperative motion of cellulose chain segments and absorbed water molecules. The a2 transition can be separated into two distinct relaxations (a2,1 and a2,2) that are caused by micro-Brownian motions of amorphous chain segments [142–144]. The a1 transition is associated with motions in the non-crystalline regions as the crystalline segments are constrained. However, as the a1 transition also coincides with the onset chemical decomposition of the molecule due to the high temperature, it is possible that released chain segments from the crystalline regions also contribute to the relaxation [145]. The viscoelastic properties of ACCs have been investigated using dynamic mechanical analysis (DMA) [13, 69, 95, 97, 98]. The storage modulus (E0 ) of the composites formed with LiCl/DMAc as solvent system are reported to be lie between 1010 and 1011 Pa between temperatures of -150 and 0 °C. E0 only gradually decreases with increasing temperature as a consequence of strong hydrogen bonding in cellulose I [95, 97, 98]. At temperatures above 250 °C, the cellulose composites start do degrade, resulting in severe drop in E0 . The decrease in E0 at higher temperatures than 250 °C of those composites goes in hand with the reported a2 transition of regenerated cellulose. Optical properties Optically transparent cellulose-containing composites based on bacterial cellulose, epoxy and acrylic resins have been produced; according to Nogi et al. [146], the optical transparency is due to the dimensions of the bacterial cellulose ribbons lying in the nanometre range. However, ACCs have also been observed to be optically transparent under certain conditions as first reported by Gindl et al. [71] and attributed to the lateral dimension of the used microcrystalline crystals of 1–3.5 nm. ACC films based on
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the LiCl/DMAc solvent system show variations in optical transparency as a function of the dissolution time, with increasing time resulting in greater transparency (Fig. 8). As the dimensions of the used cellulosic materials in other studies exceed the nanometer range, the transparency of those ACCs has been attributed to the good adhesion between reinforcement and matrix phase and closure of internal pores in the cell wall [95, 97].
Derivatized all-cellulose composites Introduction The field of derivatized cellulose, of which much is beyond the scope of this article, has been reviewed elsewhere [147– 152]. One of the first processes using cellulose derivatives was the viscose process, discovered by British chemists Charles Cross, Edward Bevan and Clayton Beadle in 1891 [153]. The expression ‘‘Viscose’’ originates from the highly ‘‘viscous cellulose’’ solution obtained during the dissolution process, that was later contracted to ‘‘Viscose’’. A major drawback of this process is the contamination of the wastewater by carbon disulfide and other polluting sulphur by-products during cellulose derivatization. This is one of the main reasons that many improvements have been made to the process and the resulting products such as rayon fibre in the last century to improve fibre quality and reduce the environmental impact [153]. The so-called man-made cellulose fibres have shown a steady rate of production in the range of 2,500 to 3,000 kt/year in the last decades with the main application being textile fibres [154]. Three of the most recent approaches for the production of thermoplastic ‘‘all-plant fibre composites’’, selfreinforced cellulose composites and cellulosic nanocomposites via benzylation, oxypropylation and esterification, respectively, are presented.
Benzylated, oxypropylated and esterified celluloses In the last decade, alternative approaches to form allcellulosic structures using cellulose derivatives have been undertaken—we term these materials derivatized ACCs. The first approach involves the production of all-plant or all-wood fibre composites by a benzylation treatment of the cellulose source [155, 156]. This is based on a Williamson synthesis reaction involving nucleophilic substitution of an oxide or a phenoxide ion for a halide ion Wood-OH þ NaOH ! Wood-O Naþ þ H2 O Fig. 8 Optical transparency of ACCs as a function of dissolution time in DMAc/LiCl. Reprinted from Ref. [97]. Copyright 2008, with permission from Elsevier
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Wood-ONa þ ClCH2 -C6 H6 ! Wood-O CH2 C6 H6 þ NaCl
ð1Þ
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where, Wood-OH represents the hydroxyl groups mainly present in cellulose [157, 158]. The matrix material is swollen in NaOH and afterwards transferred to the benzyl chloride. The solution is stirred for several hours at temperatures above 100 °C. It is then washed to remove inorganic salts, benzyl chloride and its by-products to yield a liquid matrix phase consisting of the used cellulosic material. Due to the complexity of wood and its macromolecules, the extent of benzylation is measured indirectly by observing the weight gain as the modification proceeds. Benzylated fibres show the formation of a thermoplastic region surrounding the fibre core [157–160]. The composites can then be simply formed by hot-pressing [157– 160]. Fibre volume fractions of up to 40% have been achieved using this method. Lu et al. reported that the rate of benzylation is hardly affected by the wood source, but depends strongly on the amounts of NaOH and benzyl chloride (C6H5CH2Cl) as well as the processing temperatures and reaction times [157–159, 161]. Increasing the degree of benzylation of cellulosic material leads to a decrease in crystallization due to disruption of the hydrogen bonds within the cellulose molecules, also reducing the final mechanical properties. This is explained by the larger benzyl molecule introducing a larger free volume and causing a change in the supramolecular structure of the molecule [157–159]. It has been shown that it is also possible to produce ‘‘allplant fibre composites’’ by an oxypropylation treatment of cellulosic fibres as first reported by Gandini et al. in 2005. This is done by using a Brønsted base to activate the hydroxyl groups of the cellulose followed by an anionic polymerisation of the PO in a ‘‘grafting from’’ process. The fibres are first immersed in a solution of ethanol and potassium hydroxide for several hours. After the alcohol has evaporated, the fibres are then mixed with propylene oxide (PO) under nitrogen atmosphere in an autoclave at temperatures of 130–150 °C. The PO homopolymer created by chain-transfer reactions can be removed by a Soxhlet extraction using hexane. This effectively grafts a thermoplastic polymer matrix onto the outer surfaces of the fibres, with the amount measured by the weight gain [162– 164]. In this reaction, cellulose I is converted to an oxypropylated amorphous derivative. de Menezes et al. [163] report that the amorphous regions of the cellulosic materials are more prone to modification than crystalline regions, while the degree of modification is dependent on the amount of PO used. With increasing amounts of PO, the crystalline regions will also take part in the reaction although increased degradation of the cellulose structure and reduction in mechanical properties can occur. As for benzylated cellulose, oxypropylated cellulose can be hotpressed to form a composite film, with fibre fractions ranging from 10 to 40 vol.% [157–160, 162–164].
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Matsumura et al. produced cellulose nanocomposites by partial esterification of wood pulp by a treatment with a p-toluene sulphonic/hexanoic anhydride system using a cyclohexane based reaction medium that caused no swelling of the cellulose fibres. To produce the composites, pulp fibres were exposed to a hexanoylation reaction to receive heterogeneously hexanoylated pulp fibres. The hexanoylation was assumed to start from both the surface of individual microfibrils and unordered regions within the fibre followed by hexanoylation of the microfibril core. Those fibres were mixed with water or methanol and filtered afterwards to receive a uniform fibre mat in disc shape. The discs were compression moulded at 155–170 °C and at room temperature. The so produced thermoplastic composites of unmodified cellulose I and esterified cellulose were semitransparent [165, 166]. The degree of benzylation is decisive for the composites’ properties. A higher presence of benzyl groups is reported to increase the viscosity of the material. This can lead to insufficient wetting of the fibres and result in a weak interface that reduces the flexural and tensile strength at a fibre volume share of 40% as the fibre are not completely surrounded by the matrix phase [158, 160]. An overview of the tensile properties of benzylated and oxypropylated ACCs is given in Table 4. The thermomechanical and viscoelastic properties of benzylated ACCs have been analysed with thermomechanical analysis (TMA) [158, 159]. Benzylated ACCs exhibit thermoplastic behaviour due to the benzylated cellulosic material with a softening temperature in the range of about 90 to 120 °C depending on the source of cellulose and degree of benzylation. Lu et al. [159] suggested that, as the plastification does not change the chemical backbone of the cellulose molecule chains, that softening should not be attributed to the a1 transition of the cellulose but could rather be caused by inter-molecular slips. Analogous, for oxypropylated cellulose, the amount of PO used is a determining factor for the tensile properties, as the use of higher amounts leads to a decrease in strength and stiffness but an increase in elongation to break. Those changes are likely to be caused by an increase of the thermoplastic amorphous phase [162]. For the esterified composites, the degree of substitution (DS) during the hexanoylation reaction is the decisive factor for the mechanical performance. Similar to oxypropylated cellulose, elongation increased with DS, while strength and stiffness decrease at highest tested DS of 2.0 [166]. Comparing the reported tensile properties of non-derivatized ACCs with those of benzylated, oxypropylated and esterified ACCs shows that the biggest differences seem to be in tensile strength. The strongest non-derivatized ACCs are about five to ten times stronger than their derivatized
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been investigated in detail. This will be necessary to judge the effectiveness of the solvent and therefore allow a classification of the different processing methods, involved costs and resulting material properties. The costs of the solvent could be minimised if it can be recovered and recycled, as it is the case for NMMO and ILs, assuming the recycling process is not too cost-intensive. On the other hand, in case of a non-recyclable and possibly hazardous solvent, the costs of its disposal must be taken into account. Another important factor is the amount of dissolvable cellulose and resulting necessary amount of solvent. It is also thinkable that different solvents will only work with cellulose sources of low DP and crystallinity whereas others might be the ideal choice for high-class raw materials. Furthermore, more research is necessary to know how the different solvent systems interact with the other components such as hemicelluloses and lignin of financially attractive cellulose sources like natural fibres and wood in contrast to highly purified cellulosic materials such as microcrystalline cellulose or filter paper. The interfacial properties between the cellulosic matrix and reinforcement has not been analysed with the typical tests developed for glass- or carbon fibre-reinforced composites such as pull-out or single-fibre fragmentation tests. Hence, no values of the interfacial shear strength of those composites have been published. This is somewhat surprising, as the improved interface seems to be the driving factor behind the development of ACCs. So far, the quality of the fibre-matrix-interface has only been judged using SEM pictures. A specific analysis of the interfacial properties of ACCs is necessary to verify the proposed positive effect of a monocomponent composite and verify a possible connection to the improved tensile strength. Understanding the formation, structure and quality of the interface will be essential for the production of predictable composite materials.
counterparts, while the Young’s moduli and strain at failure are in a similar range. Based on the reported values of filter paper in Tables 2 and 4 one can assume that the nonderivatized formation of the composite using LiCl/DMAc leads to better tensile properties than the oxypropylation, but without a comparative study the influence of processing on composite properties are not evaluable. Therefore, the biggest difference seems to be the thermoformability of the derivatized ACCs.
Future challenges and applications for all-cellulose composites In spite of the promising properties of ACCs, there still remains much research required to understand the fundamentals of these materials and to find suitable industrial applications. The hydrophilicity of cellulose will require additional processing to avoid swelling and degradation in long-term applications. Even while processing, hydrophilicity of the cellulosic raw materials can cause a significant amount of swelling in the ACCs and a resulting amount of shrinkage after drying. It would be interesting to see how the swelling and shrinkage correlate with the amounts and types of cellulose used and the applied processing steps. Unfortunately, this subject has not been addressed by the researchers so far, as predictable and controllable shrinkage will be essential for industrial scale production of those composites. Although several research papers have been published on cellulose dissolution with ILs for different applications, only few research groups have focused on using ILs to process composite materials. The same is true for ACCs processed with NMMO and NAOH/urea. Until now, the effect of the different solvents on the mechanical properties of single cellulosic fibres or the resulting ACCs has not
Table 4 Overview of isotropic (ISO) and unidirectional (UD) ACCs based on cellulose derivatives produced by benzylation, oxypropylation and esterification Cellulose source
Reinforcement
Treatment
Fibre volume fraction (%)
Tensile strength (MPa)
Young’s modulus (GPa)
Strain at failure (%)
Reference
Wood sawdust
Sisal fibre (UD)
Benzylation
30
90
15
4.8
Lu et al. [158]
Wood sawdust
Sisal fibre (UD)
Benzylation
40
68
20
4.2
Lu et al. [158]
2.1
Wood sawdust
Sisal fibre (ISO)
Benzylation
15
32
2.35
Sisal fibre
UD
Benzylation
–
43
3
Filter paper
ISO
Oxypropylation
–
18.7
1.18
2.7
Filter paper
ISO
Oxypropylation
–
25.7
1.31
4.91
de Menezes et al. [162]
Wood pulp
ISO
Esterification
–
25
0.8
6
Matsumura et al. [166]
Wood pulp
ISO
Esterification
–
20
1.3
5
Matsumura et al. [166]
The cellulose sources, reinforcements, fibre volume fractions and tensile properties are shown
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It has already been mentioned that so far the structure of those composites is limited to a simple two-dimensional geometry. Compositions that are more complex would allow competition with other fibre-reinforced polymers. As well as structural applications, it could also be possible to use ACCs for biomedical applications. Materials made from bacterial cellulose [39, 167] and other cellulosic materials have successfully been tested for such applications [41, 168–170]. The good mechanical properties of the ACCs, therefore might allow an application as substitution of bone or cartilage material [54]. Another application could be so-called ‘‘smart materials’’ in which regenerated cellulose [171] and cellophane [172] are used as electro-active paper. As those can be used for a broad variety of applications such as sensors, electrical displays and micro robots [173], the improved mechanical properties of the ACCs might provide an attractive alternative to mere cellulose paper. The ACCs produced so far have been produced in laboratory scale experiments and little is known on their processability with mass manufacturing methods. As so far all those composites had a film or sheet like structure, blow moulding seems to be one applicable method as it is successfully applied for NMMO-solutions [174]. Irrespective of the manufacturing method, the production costs of the ACCs will depend on several other aspects. The quality of the cellulose source will play a major role in the calculation. Highly purified cellulose or other necessary pretreatments such as excessive drying of the raw materials will increase the costs of the composite whereas the use of low cost raw materials, such as waste products of, for example, the wood or textile industry will reduce the expenses. A further aspect that must be mentioned is whether ACCs can really be considered a ‘‘greener’’ alternative to existing materials. The example of the viscose process shows, that using biological raw materials and biodegradation of the end product count little when the processing involve highly hazardous materials [175]. The ecofriendliness of the ACCs will therefore strongly depend on the involved materials and processing methods and the corresponding energy needs. It will be necessary to do environmental audits on the composites and their production as soon as they are ready for industrial fabrication to judge how ‘‘green’’ those materials really are. In addition to that degradation studies on those composites might provide important information as well.
Conclusions All-cellulose composites are an interesting new development in the area of green and biocomposites. Their superior tensile properties compared to other biocomposites and the
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suggested improved interfacial properties are especially remarkable. The broad variety of possible cellulose resources, cellulose solvents and the different processing possibilities promise a wide range of materials for various applications. However, a manufacturing process has to be found that can be easily controlled and can be used for different cellulose sources. It would be beneficial if such a process could be at least partially automated and adapted to already existing polymer processing methods. That way, the transformation from laboratory to industrial scale will become a lot easier. However, before that can be done a lot of work is still required to characterise and fully understand those composites. Many aspects, for example the different phases of cellulose within the composites and their relation to mechanical and thermal properties need closer investigations. Furthermore, it has to be determined how the raw materials, cellulose solvents and processing methods influence the material properties of the composites. Impressive as the mechanical properties of the ACCs are, it will be necessary to identify the single aspects that might be responsible for those properties. A closer inspection of the interface between reinforcing material and matrix in the ACCs is necessary to explain the benefits of monocomponent composites. A real classification of this new class of materials is surely premature at this stage, but nonetheless it can be said that, based on the results reported so far, ACCs could play an important role in the area of biocomposites in the future. Acknowledgement The authors acknowledge the financial support of the New Zealand Foundation for Research, Science, and Technology.
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