Hamayoun
Mahmood
a,
Muhammad
Moniruzzaman
*ab,
Suzana
Yusup
a and
Tom
Welton
c
aDepartment of Chemical Engineering, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, Perak Darul Ridzuan, Malaysia. E-mail: m.moniruzzaman@utp.edu.my
bCenter of Research in Ionic Liquids, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, Perak Darul Ridzuan, Malaysia
cDepartment of Chemistry, Imperial College London, London, SW7 2AZ, UK
First published on 13th April 2017
In recent years, the utilization of renewable resources, particularly lignocellulosic biomass based raw materials, to replace synthetic materials/polymers for the manufacture of green materials has gained increased worldwide interest due to growing global environmental awareness, concepts of sustainability and the absence of conflict between food and chemical/materials production. However, structural heterogeneity and the presence of networks of inter- and intra-molecular interactions in biopolymer matrices remain unsolved challenges to clean pretreatment for biocomposite processing. A number of techniques including physical, physico-chemical and chemical methods have been investigated for the pretreatment of renewable resources. Most of these methods require high temperatures and pressures, as well as highly concentrated chemicals for the pretreatment process. Fortunately, ionic liquids (ILs) – potentially attractive “green” recyclable alternatives to environmentally harmful organic solvents – have been increasingly exploited as solvents and/or (co)solvents and/or reagents for biopolymer processing. Compared to conventional approaches, ILs in processing biodegradable composites exhibit many advantages such as being noncorrosive and nonvolatile, having excellent dissolution power under relatively mild conditions and high thermal stability. Presently, a wide range of different approaches have been explored to further improve the performance of ILs processing of biobased polymers for composites manufacturing. The main goal of this review is to present recent technological developments in which the advantages of ILs as processing solvents for biopolymers for the production of a plethora of green composites have been gradually realized. It is hoped that the present article will inspire new ideas and new approaches in ILs-assisted processing of renewable resources for green composite production.
The use of petroleum based non-biodegradable plastics and composites and their existing processing techniques have become recognized as a threat to the environment and consume much limited fossil resources.27 The UN conference on climate change in Copenhagen December 2009 evoked a fierce public debate about the future of the earth and the need for a transition towards a CO2 neutral biobased economy (bioeconomy) was emphasized.28 This situation has led people from both academia and industry to focus on the development of new eco-friendly materials based on biobased renewable resources. In contrast to the field of renewable energy, in which biobased energy resources are in competition with wind and solar energies, the only renewable alternative to materials based on carbon petrochemistry is biobased raw materials. To this end, polymeric carbohydrates, e.g., cellulose, starch, chitin, inulin, chitosan, lignin etc. are found abundantly in nature as structural building elements.29 Agricultural industries generate a substantial amount of lignocellulosic agricultural waste every year, which is mainly composed of cellulose, hemicellulose and lignin. The typical percentages of dry weight of these wastes are 35–50% cellulose, 20–35% hemicellulose and 5–30% lignin.30 Thus, the utilization of non-food lignocellulosic waste from various agro-industries for the production of high value composite materials could be an attractive approach instead of creating pollution problems.31
Sustainability in composites science depends upon not only the assortment of renewable, or environmentally benign raw materials for their manufacture, but also on the development of mild pretreatment methods that avoid the use and production of hazardous substances to minimize their environmental impact.32 The critical challenge to extend the novel applications of biopolymers and lignocellulose for manufacturing biocomposite materials is to overcome the strong inter- and intra-molecular hydrogen bonding which lead to its recalcitrant nature.26 These attributes assure the excellent chemical and physical stability of polysaccharides but also prevent their widespread utilization.33,34 Consequently, considerable efforts have been devoted to enhance the processability of these carbohydrate polymers and lignocellulosic materials. For example, the ‘Viscose process’ is a 100 year old technique for cellulose processing which involves derivatizing the cellulose with carbon disulphide to cellulose xanthate followed by dissolution in sodium hydroxide.35 Alternatively, the ‘Lyocell process’ employs the solvent N-methylmorpholine N-oxide (NMMO) for direct dissolution of cellulose in an industrial fiber-making process.36 Nevertheless, each of these technologies carries a significant drawback; considerable byproduct formation in cellulose/NMMO/water systems can cause detrimental effects such as degradation of cellulose, temporary or permanent discoloration of resulting fibers, pronounced decomposition of NMMO,37 and the Viscose process generates two kilograms of waste per kilogram of cellulose obtained.38 In addition, various pretreatment technologies have been developed for lignocelluloses which apply hydrothermal or chemical treatments after mechanical comminution. Hydrothermal techniques include steam explosion,39 carbon dioxide explosion,40 and hot water treatment,41 whereas chemical processes can be dilute-acid treatment,42 alkali treatment, the organosolv process using organic solvent,43 ammonia fiber explosion and ozonolysis.44 However, most of the above mentioned pretreatment methods feature several drawbacks. They have to be tailored to the specific biomaterial source and can cause significant decomposition of biopolymer components to side-products, which can severely inhibit further processing of the biocomposite.45 Additionally, some pretreatment methods require extreme conditions of temperatures and pressures or strong acids or bases for which special equipment are necessary. The perceived adverse effects of such processes combined with their safety and environmental issues, has resulted in increased pressure to minimize their use, particularly in the light of recent regulations that have aggressively targeted the reduction of the emission of industrial pollutants.46 Thus sustainability, green chemistry, and eco-efficiency are not just newly coined buzzwords, but are forming the principles that are directing the development of the next generation of industrial chemical operations. Subsequently, novel processing technologies for widespread potential applications of biobased polymeric carbohydrates and lignocellulosic agricultural waste are being extensively investigated.47,48
The technological utilization of natural polymers and lignocellulosic materials for biocomposite manufacturing can potentially be enhanced significantly by their dissolution in ILs rather than in traditional organic solvents because of ILs’ unusual solvent properties.26,49 Studies on the dissolution of different polysaccharides in ILs over the last 10–15 years have shown that by using these solvents efficient selective extraction of the components is possible.50 In addition, many biopolymers and lignocellulosic fibers after regeneration from their IL solutions exhibited high thermal and operational stability.51,52 Therefore, ILs are receiving increased interest as new and highly effective dissolution media for a wide range of biodegradable polymers and lignocellulosic materials. The main objective of this review is to present a brief overview of the current state-of-the-art on the role of ILs as dissolution media for various polysaccharides for engineered green materials applications.
Ionic liquids are generally considered to be capable of having a broad range of intermolecular interactions such as hydrogen bonding, dipolar, dispersive and ionic.57,58 Therefore, various compounds are remarkably soluble in ILs. Moreover, many compounds which are insoluble or only partially soluble in other organic solvents can be efficiently dissolved in ILs possessing coordinating anions such as [OAc]−, Cl−, [NO3]− that are strong hydrogen bond acceptors.59,60
Based on their solvation capabilities, ILs are classified generally as highly polar solvents. Various methods have been employed to understand the polarity of ILs such as partition,5 fluorescence probe methods61 and solvatochromic dyes.62 In addition, a number of empirical and semi-empirical polarity scales such as COSMO-RS,63 the Hansen solubility parameters,64 and the Kamlet–Taft polarity method5 have been applied to predict and correlate the solubility of biopolymers and biomolecules in ILs with their polarities. In the Kamlet–Taft system, the parameter describing the hydrogen-bond basicity is expressed by β, whereas α represents the hydrogen-bond acidity and π* is the measure of interactions through polarisability and dipolarity effects. In ILs having non-functionalised cations like dialkylimidazolium, the β value is primarily affected by the anion.65 For cellulose dissolution in ILs, it was found that dissolution capabilities of ILs are characterized by their high values of hydrogen-bond basicity parameters.24
Ionic liquids are usually miscible with polar solvents such as ketones, lower alcohols and dichloromethane, but immiscible with non-polar organic solvents including ethers and alkanes. Furthermore, on the basis of solubility of ionic liquids in water they can be classified into hydrophilic (water miscible) and hydrophobic (forming a biphasic system with water). It was found that water miscibility of an IL also generally depends on its anion.66
ILs have higher viscosities as compared to those of most common molecular solvents. In general, the viscosities of ILs depend on their interionic interactions, such as Coulomb forces, hydrogen bonding and van der Waals interactions. Therefore, viscosities of IL vary considerably with composition, temperature and chemical structure. The higher viscosities of ILs significantly hinder the dissolution of lignocellulose and other biopolymers in these. In order to reduce viscosities, organic co-solvents including dimethylsulfoxide, dimethylformamide and 1,3-dimethyl-2-imidazolidinone etc. have also been successfully used.67Fig. 1 provides structures of some ILs that have been used for processing of various biopolymers and lignocellulosic materials for biocomposite manufacturing.
Fig. 1 Structures of the ILs used for dissolution of biopolymers and lignocellulosic materials for fabrication of biocomposites: (a) [C2C1im][OAc], (b) [C4C1im]Cl, (c) [(C1C2)C1im]Br, (d) [C2C1im]Br, (e) [(C1C2)C1im]Cl, (f) [C2C1im][Et2PO4], (g) [C4C1im][BF4], (h) [C4C1im][OAc], (i) [C4C1im][PF6].24,93 |
As ILs are tunable solvents, they can be designed by appropriate combination of cations and anions for a particular biopolymer, which is not possible using conventional organic solvents. In fact, since ILs can undergo a wider spectrum of intermolecular interactions, they are capable of dissolving a vast range of biopolymer compounds that are insoluble in organic solvents.68 For example, ILs can be used for dissolution of starch,69 chitin,70 chitosan,71 polylactic acid,72 lignin,73 cellulose,22 or for even complete dissolution of lignocellulosic materials.24,74 ILs can also be employed for selective dissolution and extraction of any desired or undesired component from a mixture or matrix solution.75
In comparison to molecular solvents, the remarkable benefit of using ILs for dissolution of lignocellulose and various biopolymers is that they can dissolve these under relatively mild operating conditions and at normal atmospheric pressure. Fukaya et al.76 reported the dissolution of cellulose at the temperature of 45 °C for 30 min by utilizing alkylimidazolium based ILs with a methyl phosphonate anion.
As well as acting as dissolution medium, recently many ILs, especially with methylimidazolium cations, were found to outperform other conventional plasticizers by significantly disrupting the inter- and intra-molecular hydrogen bonding within polysaccharides, such as starch, and thus can be employed as excellent media for polysaccharide modification and plasticization.77,78 Sankri et al.79 and Leroy et al.80 have performed pioneering work using the IL 1-butyl-3-methylimidazolium chloride ([C4C1im]Cl) as a novel plasticizer in melt processing of starch-based polymers with remarkable improvements in plasticization, hydrophobicity and electrical conductivity being reported.
Pretreatment is an essential unit operation in a polysaccharides based biorefinery and for sustainable material applications, but is considered to be of one the most cost intensive operations. The production of valuable products from lignocellulose or starch has a narrow profit margin. Various acidic, alkaline, hydrothermal and ammonia fiber expansion (AFEX) pretreatment technologies have been examined under high biomass loadings (>15%).81,82 Lately, ILs have been shown to be highly efficient dissolution media for lignocellulosic materials under solid biomass loads of as high as 50%.83 Recently, we have explored high solids loading, because of lower overall cost, improved efficiency and environmental benefits.51 Thus, the possibility of high-throughput pretreatment in a continuous process could improve the potential for the use of ILs as a cost-effective and extremely promising technology for manufacturing sustainable composites from various polysaccharide raw materials. The most important advantages of ILs as dissolution media for the fabrication of polysaccharides based biocomposites is the possibility of ILs recycling and product recovery schemes which are not viable with conventional organic solvent systems.74,84
It should be noted that biopolymer dissolution is often controlled by kinetics rather than by thermodynamics. Sescousse et al.85 performed a comparative study for the dissolution and regeneration of cellulose in the ILs 1-butyl-3-methylimidazolium acetate [C2C1im][OAc] and [C4C1im]Cl with conventional cellulose solvent systems, i.e., NaOH and N-methyl-morpholine N-oxide (NMMO). Kinetic study of the cellulose regeneration process from cellulose–IL solution revealed that this was a diffusion-controlled process. Gavillon and Budtova86 also predicted that the cellulose regeneration steps from cellulose–NaOH–water and cellulose–NMMO solutions were diffusion-controlled. For complete dissolution of a biopolymer, the solvent must diffuse into the structure of polymeric chain to affect both the amorphous and the crystalline regions. Therefore, high diffusivity and the capability of chain disentanglement are necessary attributes for an effective solvent.87 At a given cellulose concentration, the diffusion coefficient for 1-ethyl-3-methylimidazolium acetate [C2C1im][OAc] and [C4C1im]Cl were four to five times lower than the diffusion coefficient of NaOH and about twice lower than that of NMMO, which might be due to the larger size of the ILs.86–88 Moreover, compared with the NMMO process, the direct dissolution of cellulose in IL is more easily controlled and the process is inherently safer. It has been described that cellulose fibers prepared using ILs displayed similar properties in terms of elasticity and tenacity compared to the fibers manufactured by the Viscose and Lyocell processes. In addition, the process can be designed such that both fibrillating and non-fibrillating fibers can be fabricated specially for textile applications.89,126,127
Ionic liquids exhibit a far lower degradation potential compared to the NMMO. The study of Wendler et al. showed that a [C2C1im][OAc]–cellulose solution exhibited a Tonset (temperature corresponding to the point of intersection of tangents to two branches of the thermogravimetric curve) of 180 °C compared to the value of 146 °C for a NMMO–cellulose system. Apart from Tonset, numerical estimation of pressurization rates from pressure curves of cellulose–solvent mixture showed that compared to NMMO the pressure rise during the exothermic event for cellulose–IL solution was much less, which evidently suggested the ILs as solvents with much higher thermal stability.90
The dissolution of cellulose in acidic solvents is accompanied by a substantial hydrolysis of cellulose and causes depolymerization, which consequently limits its use in some applications.91 On the contrary, for cellulose dissolution in 1-allyl-3-methylimidazolium chloride [(C1C2)C1im]Cl at 60 °C, a well resolved 13C NMR spectrum of all the six anhydroglucose units was observed, indicating that the cellulose dissolved in a similar manner in [(C1C2)C1im]Cl as in other familiar solvents for cellulose including sodium hydroxide–carbon disulfide solution.92 Allyl- and n-alkyl-imidazolium based ILs with Cl−, Br− and [OAc]− anions showed 5–14.5% cellulose solubility over a temperature range of 80–110 °C.93 In comparison, an aqueous mixture of NaOH/urea/thiourea that could readily dissolve cellulose exhibited a maximum cellulose solubility of only 7.2%.94 Moreover, thermal gelation of cellulose in aqueous NaOH/urea/thiourea solution due to self-aggregation of the cellulose chains has also been reported.95
Although a great deal of experimental and computational advances have been made, there is no explicit understanding about the specific mechanism of biopolymer dissolution in ILs compared to other solvents. Currently, two main models for the ILs’ interactions with cellulose prevail: (1) the dissolution process is controlled by the interaction of the anion with the biopolymer with no particular role for the cation; and (2) the principal driving force for cellulose dissolution originates from the H-bond interactions of the cellulose hydroxyl group with both the cation and the anion of the IL.96 Conversely in the case of the NMMO solvent, the proposed mechanism presumes the cleavage of intermolecular bonds of cellulose via the creation of a soluble complex of stronger hydrogen bonds between the NO group of NMMO and the cellulosic hydroxyl groups.97 For mixed inorganic/organic solvents such as lithium chloride/dimethylacetamide (LiCl/DMAc), the dissolution mechanism is suggested to go through an intermediate involving the interaction of Cl− (due to its basicity) with cellulose.98 This type of interaction can be considered as a polyelectrolyte effect, in which polymer molecules are forced apart because of charge repulsion. A similar polyelectrolyte effect has been suggested for the tetrabutylammonium fluoride/dimethyl sulfoxide (TBAF/DMSO) solvent system.99
Fig. 2 Schematic representation of the manufacture of various biodegradable composites based on ILs dissolution of biopolymers and lignocellulose. |
The IL [(C1C2)C1im]Cl was used as a single component solvent for cellulose to fabricate the regenerated cellulose material with good mechanical properties by a solution casting method.106 The cellulose samples (cotton linters) were cut into smaller pieces and dried at 70 °C for 3 h in a vacuum oven. A specific mass of cellulose was dispersed into 20 mL of ([(C1C2)C1im]Cl) to get 4% polymer concentration, and the mixture was continuously heated and stirred until the material was completely dissolved. Finally, the solution was cast onto a glass plate to achieve a thickness of about 0.50 mm, air bubbles were removed in a vacuum oven and then a transparent regenerated cellulose film was formed in a water bath. The thickness of the film was controlled to within 0.5 mm to avoid curls in the cellulose films. The regenerated cellulose film was washed with plenty of distilled water and dried at 60 °C in a vacuum oven. Thus, [(C1C2)C1im]Cl based processing of cellulose was proposed to be a promising “green process” for the fabrication of regenerated cellulose films, which can solve the inherent environmental problems of the generation of waste toxic gases formed during the current industrial processes. Dissolution and regeneration of cellulosic biofilm from [C4C1im]Cl was also reported by Liu et al.,107 where cotton pulp was used as the raw cellulose source. The authors observed that [C4C1im]Cl was a direct solvent for cellulose and its solubility could reach up to 13 wt% at 90 °C in 7 h, but at the same time the degree of polymerization was remarkably reduced. Takegawa and coworkers108 prepared the bi-component biopolymer film containing cellulose and chitin each dissolved separately in [(C1C2)C1im]Br and [C4C1im]Cl respectively at 100 °C for 24 h with continuous stirring to give clear solutions of chitin and cellulose. The stress–strain curves were also measured under tensile mode and it was concluded that the biofilms became more elastic with decreasing the ratio of chitin to cellulose in the final product. Meng et al.109 successfully dissolved and regenerated the native skin collagen in [C4C1im]Cl (Fig. 3). However, the triple helical structure of the collagen had been partly destroyed during the dissolution and regeneration process. A possible mechanism of dissolution and regeneration of collagen in IL was also proposed and it was deduced that the IL dissolved the collagen fibers by mainly breaking the hydrogen bonds and the ionic bonds in collagen macromolecules. Regenerated wool keratin films were prepared from wool keratin/ionic liquid solutions through the addition of water, methanol or ethanol as coagulation solvents.110 It was suggested that [(C1C2)C1im]Cl had higher solubility for wool keratin fibers than that of [C4C1im]Cl. XRD data also confirmed that the regenerated films exhibited a β-sheet structure and the disappearance of the α-helix structure. Further, Byrne et al.111 reported the fabrication of regenerated film of three natural polymers, raw cotton, silk and wool using [(C1C2)C1im]Cl at 105 °C. The new biocomposite films showed enhanced thermal stability compared to single component films, which was attributed to the increase in intra molecular hydrogen bonds for the biofilms. Similarly, Rahatekar et al.112 successfully combined the biocompatible properties of chitin with the high electrical conductivity of carbon nanotubes (CNTs) by mixing these using an imidazolium-based ionic liquid as a common solvent/dispersion medium. The IL allowed the uniform dispersion of CNTs and dissolution of chitin to create a biocompatible, electrically conducting scaffold permissive for mesenchymal stem cell function. Recently, Byrne and co-workers113 presented an exciting example of using [(C1C2)C1im]Cl for the preparation of a novel regenerated cotton/duck feather composite film. The new blended films showed enhanced elastic properties as well as thermal stability in comparison to the single component films regenerated from the same IL. The amount of α-helix in the composite film was responsible for improvement in the elastic properties of the fabricated films.
Fig. 3 Schematic representation of collagen/cellulose composite materials preparation using IL [C4C1im]Cl.109 |
The novel hybrid green composite films comprising of cellulose, starch and lignin have been prepared by respective dissolution in [(C1C2)C1im]Cl.69 The experimental results showed that the relative contents of cellulose, starch and lignin had a significant impact on the mechanical properties of composite films. The composite films exhibited good thermal stability and high gas barrier capacity with a CO2:O2 permeability ratio close to 1.
Fig. 4 An illustration of a typical electrospinning apparatus for biofiber processing by ILs.118 |
Polaskova et al.119 used a wet electrospinning technique for the transformation of raw pine wood into microfiber (1–4 μm) by dissolution in [C2C1im][OAc] or [C2C1im][lactate]. It was reported that 5% wood concentration in the IL was most appropriate for electrospinning and an increase in the biomass loading up to 10% complicated the process due to drastic increase in the viscosities of the solutions. Raw delignified lignocellulose biomass (hemp) was successfully electrospun using ([C2C1im][OAc]) as the spinning solvent by Ahn et al.120 As expected, the spinning efficiency and fiber morphology strongly depended on the lignin contents of the raw biomass. It was observed that when the lignin content was higher than 6%, no fiber was formed and the solution was converted into large droplets at the end of the nozzle. Jiang et al.121 studied the microstructure and crystalline properties of the commercial cellulose fibers regenerated and processed with different solvents and technologies with synchrotron wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS). On the basis of their findings, the IL was expected to be much more efficient and flexible in cellulose dissolution and fiber-forming processes.
Quan et al.122 prepared nonwoven nano-scale fibers of cellulose regenerated from [C4C1im]Cl using electrospinning and successfully obtained a minimum diameter of the continuous electrospun cellulose fibers in the range of 500–800 nm. Qin et al.123 observed that high molecular weight and high purity chitin powder could be recovered after complete dissolution of raw crustacean shells in [C2C1im][OAc] with the yield as high as 94% (Fig. 5). Furthermore, they reported the direct fabrication of chitin fibers and films from the extract solution. High tenacity chitosan fibers with excellent strength and initial modulus were generated from dissolved chitosan in binary IL mixtures of glycine hydrochloride [Gly·H]Cl and [C4C1im]Cl with a dry-wet spinning technology.124Fig. 6 shows that the same binary system of acidic and neutral IL ([Gly·H]Cl and [C4C1im]Cl) was used to fabricate chitosan-cellulose composite fiber with 9.4 wt% chitosan.125 The hybrid-type biopolymer fiber not only had good mechanical strength but also excellent thermal stability with Tonset of 305.1 °C.
Fig. 5 Fabrication of chitin fiber from an IL solution of crustacean shells.123 |
Fig. 6 (a) Schematic representation of the dry-wet spinning technique (b) the obtained chitosan fibers.125 |
The suitability of several chloride containing ILs was tested for their cellulose dissolving and subsequent fiber-making properties by spinning the IL–cellulose solutions into water.126 The resulting fibers were asserted to belong to the class of Lyocell-fibers and showed the same or comparable characteristics to fibers obtained from NMMO solutions. Recently, cellulosic fibers were spun in a dry-jet wet spinning process from a solution in the IL 1,5-diazabicyclo[4.3.0]non-5-enium acetate ([DBNH][OAc]) into water, resulting in properties equal or better than that made from the conventional Lyocell process (tensile strength 37 cN per tex or 550 MPa).127 A spinneret aspect ratio L/D (length to diameter ratio of fiber) of 2.0 gave a better orientation than L/D of 0.2 and allowed higher draw ratios.
Another report describes the multiwall carbon nanotube (MWCNT)/cellulose composite fibers processed from solutions in [C2C1im][OAc].128 At 0.05 mass fraction of MWCNT, the fiber tensile strength increased by about 25%, strain to failure increased by 100% and modulus essentially remained unchanged. Later, Rahatekar and co-workers129 further extended the work by evaluating the effect of fiber extrusion speed and fiber winding speed on the degree of alignment and electrical properties of MWNTs/cellulose composite fiber processed by [C2C1im][OAc]. A decrease in degree of alignment was noted when fibers were spun with higher winding speed using a constant extrusion speed. More recently, the work of Rahatekar130 corroborated the manufacture of highly aligned cellulose fibers spun from optically anisotropic microcrystalline cellulose solutions in 1-ethyl-3-methylimidazolium diethylphosphate ([C2C1im][Et2PO4]). Fibers with an average diameter of ∼20 μm, a high Young's modulus up to ∼22 GPa and moderately high tensile strength of ∼305 MPa were successfully spun from an 18 wt% cellulose solution in the IL. Similarly, in the interesting study of Byrne et al.131 composite fiber comprising of cellulose and duck feather was extruded from a solution of the biopolymers in the IL [(C1C2)C1im]Cl via a wet spinning operation. The mechanical properties of the composite fiber was shown to be better than regenerated cellulose fibers alone with a 63.7% improvement in tensile strain. The hybrid composite fiber comprising regenerated raw lignocellulosic biomass (yellow pine and bagasse) from [C2mim][OAc] with a dry-jet wet spinning process under short dissolution times (10–30 min) and temperatures above the glass transition temperature of lignin was reported by Rogers et al.132 Fibers spun using the higher temperature/shorter time method were stronger than those obtained using the lower temperature/longer time method (Fig. 7).
Fig. 7 Cellulose–starch composite gel (a) and fiber (b).133 |
Li et al.140 reported a facile fabrication method of a lignocellulose aerogel by dissolution of wood in [(C1C2)C1im]Cl by a cyclic-freeze–thaw (FT) process, as shown schematically in Fig. 8. It was shown that the IL assisted FT treatment, when used as a physical cross-linking technique, can successfully form continuous 3D network structures. Later, Sescousse et al.141 fabricated an ultra-light and highly porous cellulose material which they called aerocellulose. The material was prepared via cellulose dissolution in [C2C1im][OAc] or [C4C1im]Cl and then regenerated and dried under supercritical CO2 conditions. The density of the aerocellulose was from 0.06 to 0.20 g cm−3. The “bead-like” morphology of aerocellulose from cellulose–ionic liquid solutions was similar to that of samples obtained from cellulose–NMMO monohydrate solutions. A facile preparation of a flexible gel material from a solution of cellulose in [C4C1im]Cl (15% w/w) by keeping it at room temperature for 7 days was reported (Fig. 9).142 Elemental analysis data showed that the fabricated gel was composed of cellulose, the IL, and water. Both XRD and TGA results indicated that the crystalline structure of cellulose was largely disrupted in the material.
Fig. 8 Schematic presentation of lignocellulose aerogel preparation.140 |
Fig. 9 Photographs of (a) a cellulose–IL solution gelation process, (b) the obtained gel product.142 |
Matrices based on silk fibroin show good applicability in the field of regenerative medicine, but the cocoons of Antheraea mylitta are underutilized because of the poor ability of traditional organic solvents to dissolve these. Consequently, Silva et al. investigated the solubilization and processing of degummed fibers extracted from the cocoons of silkworms into hydrogels using [C4C1im][OAc].143 The degummed silk fibers were dissolved in the IL at 95 °C with 10 wt% solid loading, as shown schematically in Fig. 10. The outcomes of the study suggested that the use of ILs for the dissolution/processing of degummed fibers derived from A. Mylitta cocoons into hydrogels could be interesting in cartilage regeneration repair strategies. Lately, a simple method for the preparation of cellulose/graphene composite hydrogels with high toughness was developed.144 The hydrogel was prepared by regeneration of the mixture of wood pulp and reduced graphene oxide from their IL solution using deionized water as an anti-solvent. This approach provides a simple and green method to compound the multifunctional properties of cellulose with the extraordinary performances of graphene based hydrogels (Fig. 11).
Fig. 10 The fabrication of silk hydrogels from mulberry and non-mulberry silk cocoons using the IL [C4C1im][OAc] as solvent. (A): Antheraea mylitta cocoons, (B): Bombyx mori cocoons.143 |
Fig. 11 (a) Images of cellulose and cellulose/graphene composite hydrogels (CGH); (b) the freeze-dried CGH; (c) the freeze-dried CGH with height of 1.50 cm and diameter of 1.40 cm supporting a 4 kg counterpoise which is more than 14800 times its own weight.144 |
Nanofibrillar aerogels were developed from cellulose, spruce wood and from mixtures of cellulose, lignin and xylan by first dissolving in [C4C1im]Cl and coagulating from IL solution using aqueous ethanol. These microporous aerogels could be readily disintegrated into fibrous or powder-like material by rubbing between one's fingers. Aerogels made from wood were much harder and possessed much more structural strength than the hybrid aerogels made from cellulose, lignin and xylan (Table 1).171
Entry | IL | Raw material | Conditions | Loading (%) | Fabrication technique | Product | Ref. |
---|---|---|---|---|---|---|---|
1 | [(C1C2)C1im]Cl | Cellulose | 100–130 °C | 4 | Solution casting | Film | 106 |
40–180 min | |||||||
2 | [C4C1im]Cl | Cotton pulp | 90 °C | — | Solution casting | Film | 107 |
[C2C1im]Cl | 7 h | ||||||
[C2C1im][OAc] | |||||||
[C4C1im][OAc] | |||||||
3 | [C4C1im]Cl | Natural wool | 100 °C | 5 | Solution casting | Film | 145 |
Cellulose | 10 h | ||||||
4 | [(C1C2)C1im]Cl | Cotton | 100 °C | 10 | Solution casting | Film | 111 |
Silk | |||||||
Wool | |||||||
5 | [(C1C2)C1im]Cl | Cornhusk | 80–120 °C | 4 | Solution casting | Film | 146 |
[C2C1im][OAc] | Cellulose | 2–12 h | |||||
6 | [C4C1im]Cl | Silk fibroin | 90 °C | 2 | Molding | Film | 147 |
Cellulose | 12 h | ||||||
7 | [C4C1im]Cl | Cellulose | 85 °C | 8 | Solution casting | Film | 148 |
Montmorillonite | 28 h | ||||||
8 | [(C1C2)C1im]Cl | Bamboo pulp | 80 °C | 6 | Solution casting | Film | 149 |
Soy protein isolate | |||||||
9 | [C2C1im][OAc] | Crustacean shells | 100 °C | 10 | Dry-jet wet spinning | Film | 108 |
19 h | Fiber | ||||||
10 | [C4C1im]Cl | Cellulose | 100 °C | 10 | Solution casting | Film | 108 |
[(C1C2)C1im]Br | Chitin | 24 h | 5 | Gel | |||
11 | [C2C1im][OAc] | Chitin | 60–120 °C | 3 | Solution casting | Film | 112 |
Carbon nanotubes | 2–12 h | ||||||
12 | [C2C1im][OAc] | Cellulose | 80 °C | 10 | Blending | Film | 150 |
Silk | 1 h | Molding | |||||
13 | [C4C1im]Cl | Skin collagen | 100 °C | 5 | Solution casting | Film | 109 |
Cellulose | 6 h | Extrusion | Fiber | ||||
Molding | Gel | ||||||
14 | [(C1C2)C1im]Cl | Wool keratin | 100 °C | — | Solution casting | Film | 110 |
15 | [(C1C2)C1im]Cl | Cotton | 100 °C | — | Solution casting | Film | 113 |
Duck feather | |||||||
16 | [(C1C2)C1im]Cl | Cellulose | 80 °C | 6 | Solution casting | Film | 69 |
Starch, lignin | |||||||
17 | [C4C1im][OAc] | Cellulose | 85–95 °C | 6 | Solution casting | Film | 151 |
Chitosan | 6 h, 3–4 days | Vacuum freeze drying | |||||
18 | [C4C1im]Cl | Cellulose | 25–110 °C | — | Solution casting | Film | 152 |
[C4C1im][OAc] | Chitosan | 4 h | |||||
19 | [C2C1im][OAc] | Hemp biomass | — | 14 | Electrospinning | Fiber | 120 |
20 | [(C1C2)C1im]Cl | Cotton | 100 °C | — | Lab spinning set-up | Fiber | 131 |
Duck feather | |||||||
21 | [C2C1im][OAc] | Pine wood | 80 °C | 5–10 | Wet electrospinning | Fiber | 119 |
[C2C1im][lactate] | 24 h | ||||||
22 | [C4C1im]Cl | Cellulose pulp | — | — | Dry-jet-wet-spinning | Fiber | 121 |
23 | [C4C1im]Cl | Cellulose | 90 °C | 5 | Electrospinning | Fiber | 122 |
30 min | |||||||
24 | [C4C1im]Cl | Chitosan | 80 °C | 5 | Dry-wet spinning | Fiber | 124 |
1 h | |||||||
25 | [C4C1im]Cl | Chitosan | 80 °C | 5 | Dry–wet spinning | Fiber | 125 |
Cellulose | 10 | ||||||
26 | [C2C1im][OAc] | Hemp biomass | 70 °C | 14 | Electrospinning | Fiber | 153 |
24 h | |||||||
27 | [C4C1im]Cl | Cellulose, starch | 100 °C | 4–10 | Dry-jet wet spinning | Fiber | 154 |
Hindered amine light stabilizer | 20 h | ||||||
28 | [C2C1im]Cl | Silk fibroin | 95 °C | 10 | Extrusion | Fiber | 155 |
29 | [dbmim][BF4N2 ] | Quantum dots elastomer | 70 °C | 1% IL | Electrospinning | Fiber | 156 |
6 h | |||||||
30 | [C4C1im]Cl | Cellulose | Microwave heating | 10 | Electrospinning | Fiber | 157 |
Heparin | 2 | ||||||
31 | [(C1C2)C1im]Cl | Cellulose | 80 °C | 1–5 | Electrospinning | Fiber | 158 |
2 h | |||||||
32 | [C2C1im][OAc] | Cellulose | RT | 8 | Electrospinning | Fiber | 159 |
[C1C1im][Cl] | |||||||
33 | [C4C1im]Cl | Wood pulp | 100 °C | 4 | Dry-jet wet spinning | Fiber | 160 |
MWCNT | 45 min | ||||||
34 | [C2C1im][OAc] | Chitin | — | 1.5 | Electrospinning | Fiber | 161 |
35 | [C2C1im][OAc] | Cellulose | 80 °C | 4–8 | Dry-jet wet spinning | Fiber | 128 |
MWCNT | 2–3 h | ||||||
36 | [C2C1im][OAc] | Cellulose | 80 °C | — | Lab-built spinning | Fiber | 129 |
MWCNT | 2 h | equipment | |||||
37 | [C2C1im][Et2PO4] | Microcrystalline cellulose | 95 °C | 7.6–18 | Dry-jet wet spinning | Fiber | 130 |
24 h | |||||||
38 | [DBNH][OAc] | Birch kraft pulp | 80 °C | 13 | Multi-filaments piston-spinning | Fiber | 162 |
75 min | |||||||
39 | [C2C1im][OAc] | Yellow pine | 175 °C | 5 | Dry-jet wet spinning | Fiber | 132 |
Bagasse | 30 min | ||||||
40 | [C4C1im]Cl | Cellulose | 100 °C | 3 | Spinning into water | Fiber | 126 |
[(C1C2)C1im]Cl | 2–4 h | ||||||
41 | [DBNH][OAc] | Cellulose | 80 °C | 13 | Dry-jet wet spinning | Fiber | 127 |
3 h | |||||||
42 | [C4C1im]Cl | Cellulose | 100 °C | 10 | Solution sandwiched between glass plates, raising by spatula | Fiber | 133 |
Starch | 24 h | Gel | |||||
43 | [C2C1im][OAc] | Cellulose | — | — | Electrospinning | Synthetic wood fiber | 163 |
Xylan, lignin | |||||||
44 | [(C1C2)C1im]Cl | Pineapple peel | 100 °C | 2.5–7.5 | Heating–cooling–freezing–thawing–washing process | Hydrogel | 164 |
12 h | |||||||
45 | [C4C1im]Cl | Microcrystalline cellulose | 80 °C | 1.5–5.5 | Chemical crosslinking | Hydrogel | 165 |
12 h | |||||||
46 | [C4C1im]Cl | Cellulose | 100 °C | 6 | Dropping solution from syringe needle | Hydrogel beads | 166 |
Collagen | |||||||
47 | [C4C1im]Cl | Cellulose | 100 °C | 15 | Keeping in RT for 7 days | Hydrogel | 142 |
24 h | |||||||
48 | [C4C1im][OAc] | Silk fibroin from the silkworm | 95 °C | 10 | Molding followed by gelation overnight | Hydrogel | 143 |
8 h | |||||||
49 | [C4C1im]Cl | Cellulose | 80 °C | 3–5 | Degassing the solution by ultrasound | Hydrogel | 144 |
Graphene | 24 h | ||||||
50 | [C4C1im]Cl | Cellulose | 70 °C | — | Solution casting | Hydrogel | 167 |
2 h | |||||||
51 | [C4C1im]Cl | Cotton cellulose | 100 °C | 8 | Solution casting | Aerogel | 168 |
24 h | |||||||
52 | [(C1C2)C1im]Cl | Nalita wood | 80 °C, 4 h | 8 | Freeze thaw | Aerogel | 140 |
53 | [C2C1im][OAc] | Cellulose | 80 °C | 3 | Molding | Alcogels | 169 |
16 h | |||||||
54 | [C2C1im][OAc] | Cellulose | 80 °C | — | Regeneration/drying in supercritical CO2 | Aerocellulose | 141 |
48 h | |||||||
[C4C1im]Cl | |||||||
55 | [(C1C2)C1im]Br | Chitin | 100 °C | 7 | Dissolution + cooling | Gel | 170 |
48 h | |||||||
56 | [C4C1im]Cl | Spruce wood | 130 °C | 7 | Solution casting followed by coagulation and high pressure cell | Aerogel | 171 |
4 h | |||||||
57 | [C4C1im]Cl | Cellulose, starch | 95 °C | 3 | Molding | Aerogel | 172 |
Zein protein, agar | |||||||
58 | [C4C1im]Cl | Agarose | 100 °C | 5 | Regeneration from IL solution | Ionogel | 173 |
Chitosan |
A novel approach to prepare cellulose-based conductive hydrogels was demonstrated by dissolution of microcrystalline cellulose (MCC) in [C4C1im]Cl (Fig. 12).165 The obtained MCC composite hydrogel showed excellent electrical conductivity of 7.83 × 10−3 S cm−1, as measured using a four-probe method. Ma et al.178 utilized the excellent selective solubility of the IL 1-(2-hydroxyethyl)-3-methylimidazolium chloride ([(HO)2C2C1im]Cl) for microcrystalline cellulose (MCC) and nanocrystalline cellulose (NCC) to fabricate green all-cellulose composite films. The ACC film was prepared by adding NCC to the MCC–IL solution. The composite film was fully biobased, biodegradable and easily recyclable, and proposed to be useful as biomaterials and food ingredients (Fig. 13b). Huber et al.176 developed a fiber-reinforced all-cellulose composite (ACC) laminate on the basis of a conventional hand lay-up method (Fig. 13a). Four layers of each natural fiber textile (linen) and man-made cellulosic textile (Cordenka rayon) were impregnated with [C4C1im][OAc]. The layers were heated under pressure to partially dissolve in the IL so that compaction of the single laminate was achieved to form a composite. The formation of the matrix phase was obtained in situ by the regeneration of the dissolved fraction of the cellulose fiber, which resulted in a composite with strong fiber–matrix interfacial properties.
Fig. 12 (a) Cellulose hydrogel and (b) swollen hydrogel.165 |
Fig. 13 (a)The fabrication process for ACC laminate by hand lay-up method,176 (b) stress–strain curve of the all-cellulose composites.178 |
Recently, in the interesting work of Reddy et al.,179 the cellulose was extracted from the Agave plant and the extracted cellulose microfibrils were successfully dissolved in [(C1C2)C1im]Cl along with a small amount of microfibrils which remained undissolved as can clearly be observed in Fig. 14. The self-reinforced regenerated cellulose biocomposite films with an average strength as high as 135 MPa were prepared from this solution. Furthermore, a comparison of an IL based all-cellulose composite with an epoxy matrix composite was reported by partial dissolution of flax and Lyocell fiber in [C4C1im]Cl.180 It was observed that ACCs fabricated from Lyocell fiber showed similar strength and stiffness but superior extensibility as compared to Lyocell fiber–epoxy composites.
Fig. 14 (a) Photograph and (b) SEM images of regenerated all-cellulose composite film.179 |
Fig. 15 An example of fiber welding. (a) Raw silk thread, welded with [C2C1im][OAc] for 5 min at 60 °C to create a solid, yet flexible band structure, (b) and (c). SEM images of (untreated) desized silk and of the welded silk band are shown in (d) and (e), respectively.181 |
Entry | Ionic liquid | Raw material | Conditions | Loading (%) | Fabrication technique | Ref. |
---|---|---|---|---|---|---|
1 | [C4C1im][OAc] | Linen | 110 °C | 50 | Hot press | 176 |
Rayon | 80 min | |||||
2 | [C4C1im]Cl | Microfibrillated cellulose | 80 °C | — | Solution casting | 191 |
20–160 min | ||||||
3 | [C2C1im]Cl | Microcrystalline cellulose | 85 °C | 3 | Solution casting | 178 |
2 h | ||||||
4 | [(C1C2)C1im]Cl | Agave fibers | 80 °C | 4 | Solution casting | 179 |
5 | [C4C1im]Cl | Lyocell fibers | 130 °C | — | Solution casting | 180 |
Flax fibers | 1 min impregnation | |||||
6 | [C4C1im]Cl | Rice husk | 100 °C | 5–10 | Solution casting | 192 |
Filter paper | ||||||
7 | [C4C1im]Cl | Cedar wood flour | 100 °C | 60 | Compression molding followed by annealing | 189 |
Bark flour | 10 min | |||||
8 | [C4C1im]Cl | Cotton fabric | 100 °C | — | Hot press followed by annealing | 190 |
hinoki lumber | 30 min | |||||
9 | [C4C1im]Cl | Mulberry wood | 80 °C | — | Twin-screw kneader followed by injection-molding | 193 |
[C4C1im][OAc] | 2–3 h | |||||
10 | [C2C1im][OAc] | Silk fiber | 60 °C | — | Natural fiber welding | 181 |
Hemp fiber | 5 min | |||||
11 | [C2C1im][OAc] | Cotton cloth | 40–80 °C | — | Natural fiber welding | 182 |
0.5–24 h |
Shibata et al.189 developed a new method for waste wood processing based on the partial dissolution of waste wood flour in [C4C1im]Cl as depicted in Fig. 16. Wood flour (WF) and bark flour (BF) derived from cedar wood were mixed with 40 wt% [C4C1im]Cl at 100 °C and the resulting mixtures were compression-molded at 210 °C in a stainless steel mold. The tensile strength and thermal properties of the biocomposites were significantly improved by the extraction of the IL from the final product and further by the annealing process. In their further work,190 Shibata and co-workers tried to improve the tensile strength and modulus of all-wood based biocomposites from dipped hinoki lumber (HL) with dimensions 50 mm × 10 mm × 2 mm in the same IL at 100 °C and the IL-impregnated HL was hot-pressed at 210 °C for 30 min (Fig. 17a). Although the tensile strength of the fabricated biocomposite was found to be lower than that of original HL, the tensile modulus of the former was significantly improved. In-depth analysis of the structural and chemical changes of wood during IL interaction is needed to explore the opportunities for using ILs in wood and wood based composite materials.
Fig. 16 Photographs of the all-wood biocomposites compression-molded at various temperatures.189 |
Fig. 17 (a) Original hinoki lumber (HL), (b) HL composite after IL pretreatment with [C4C1im]Cl,190 (c) synthetic wood films.194 |
In the study of Wen et al.,200 complete dissolution and homogeneous lauroylation of abundantly available hydroxyl groups in ball-milled lignocellulose bamboo meal was carried out in [C4C1im]Cl at 130 °C for 6 h. The thermal stability of the esterified derivatives was found to be lower than that of the unmodified bamboo meal and the rough appearance of the bamboo meal changed into a relatively homogeneous and smooth surface morphology after lauroylation. Another work that describes the development of an IL-assisted pretreatment technology for efficient conversion of chemically modified wood material into thermoplastic material was reported by King et al.201 Benzoylation and lauroylation of spruce wood thermomechanical pulp was carried out in the IL solution 4% w/w spruce in [C4C1im]Cl followed by the incremental addition of benzoyl chloride and lauroyl chloride, respectively. Subsequently, these highly substituted benzoylated and laurolyated lignocellulosic spruce fibers were used as reinforcement for thermoplastic composites of polypropylene and polystyrene. Excellent dispersion of the IL-modified wood fibers material into the synthetic polymers was achieved throughout the composite material, as highlighted by SEM images. Utilization of [C4C1im]Cl for homogeneous benzolyation of wood meal was also reported by Yuan et al.202 The focus of this study was to enhance the compatibility as well as photostability of wood with hydrophobic synthetic polymers for the production of green composites that could be used for outdoor applications. The article describes a highly promising IL based modification of wood fiber with improved properties for photostable green composites applications.
For the application of ILs in wood based composite materials, most of the studies have been focused on the effect of ILs on the bulk properties of lignocellulosic biomass and wood, and there are only a few reports related to the IL based modification of wood surface properties. In the study by Croitoru et al.,203 the effect of four types of imidazolium-based ILs, namely 1-butyl-3-methylimidazolium tetrafluoroborate ([C4C1im][BF4]), 1-butyl-3-methylimidazolium hexafluorophosphate ([C4C1im][PF6]), [C4C1im]Cl and 1-butyl-3-methylimidazolium tetrachloroferrate ([C4C1im][FeCl4]), on the various surface properties of poplar wood veneers was studied in terms of contact angle, electrical conductivity measurement and photographic image analysis and compared with those of untreated wood. It was found that IL treatment improved the flexibility of the cellulose matrix and decreased its crystallinity. Indeed, the workability of wood could be remarkably enhanced by IL surface pretreatment, which decreased its rigidity as well as preventing the accumulation of static electric charges on the surface of the wood during finishing. The changes that occurred in the structure and surface properties of maple wood veneers treated with three different types of IL, [C4C1im][BF4], [C4C1im][PF6] and 1-hexyl-3-methylimidazolium chloride ([C6C1im]Cl) were studied after being subjected to an electron beam irradiation with a 50 kGy dose.204 Also, an interesting observation was noted that a higher resistance to water penetration and spreading on the wood surface resulted after the electron beam irradiation, due to covalent bonding of the imidazolium moiety to wood structure (Table 3).
Entry | Ionic liquid | Raw material | Conditions | Modification reaction | Ref. |
---|---|---|---|---|---|
1 | [C4C1im]Cl | Bamboo meal | 130 °C | Lauroylation | 200 |
6 h | |||||
2 | [C4C1im]Cl | Southern pine Thermomechanical pulp | 70 °C | Lauroylation | 201 |
2 h | |||||
3 | [C4C1im]Cl | Wood meal | 130 °C | Benzoylation | 200 |
5 h | |||||
4 | [C4C1im]Cl | Poplar veneers | 22 °C | Surface treatment | 203 |
[C4C1im][PF6] | 15 min | ||||
[C4C1im][BF4] | |||||
5 | [C4C1im]Cl | Poplar veneers | 22 °C | Electron beam irradiation | 204 |
[C4C1im][PF6] | 15 min | ||||
[C4C1im][BF4] | |||||
6 | [C4C1im]Cl | Spruce thermomechanical pulp | 130 °C | Acylation | 205 |
[(C1C2)C1im]Cl | 4–6 h | Carbanilation | |||
7 | [C4C1im]Cl | Poplar wood | 125 °C | Acylation | 206 |
1 h | |||||
8 | [(C1C2)C1im]Cl | Cellulose | 80 °C | Acetylation | 183 |
1 h | |||||
9 | [C4C1im]Cl | Poplar wood | 130 °C | Benzoylation | 202 |
5 h | |||||
10 | [C4C1im]Cl | sspen veneer | 22 °C | Surface treatment | 207 |
[C4C1im][PF6] | 15 min | ||||
[C4C1im][BF4] |
Whether wood is used for interior applications or under exterior conditions, the important properties that could be significantly affected are its texture, color, wettability and roughness. Patachia et al.207 studied the effect of four types of imidazolium-based IL on the chemical modification and photo-degradation of the surface of wood veneers. The ILs treated wood showed some useful properties including enhanced wettability that could increase the compatibility of the wood with adhesives in fabrication of wood based composite materials.
Sankri et al.79 reported modification of starch with [C4C1im]Cl in a microcompounder and a twin screw extruder. To the best of the author's knowledge, this is currently the only article describing a continuous method of starch plasticization using ILs. The remarkable reduction in molecular weight of starch extruded with [C4C1im]Cl compared to glycerol-plasticized polysaccharide was assigned to thermo-mechanical treatment rather than interaction with IL. The use of [(C1C2)C1im]Cl as a corn starch plasticizer to fabricate solid biopolymer electrolytes has been described.215 The films were obtained by solution casting starch plasticized with 10 and 30 wt% IL respectively. Another study216 showed the plasticization of starch by mixing with same IL in the presence of lithium chloride. It was noted that thermal decomposition of starch was stimulated by high Cl− ion concentration.
Cellulose acetate is one of the most valuable cellulose derivatives, with a wide range of applications including films, coatings, membrane separation, textile and cigarette filters. Availability from renewable sources, biodegradability, non-toxicity and low cost are the combination of characteristics that make the cellulose acetate a highly promising biopolymer matrix. However, its functionalization and processing for different applications is limited by its properties such as high crystallinity and thermal decomposition temperature, which is very close to its melt processing temperature. Bendaoud and Chalamet217 utilized a micro-compounding technique to convert cellulose acetate into a thermoplastic polymer using [C4C1im]Cl. The biocomposites were thermo-molded into dumbbells and disk-shaped plates of dimensions of 30 mm × 10 mm × 2 mm and 25 mm × 2 mm (Fig. 18), respectively. It was noted that the presence of the IL strongly reduced the interactions between cellulose acetate chains as evidenced from mechanical tensile tests and rheology analyses of plasticized cellulose acetate (Table 4).
Fig. 18 Visual observations of pressed plasticized cellulose acetate samples.217 |
Entry | Ionic liquid | Raw material | Compounding method | Ionic liquid proportion (%) | Conditions | Ref. |
---|---|---|---|---|---|---|
1 | [C4C1im]Cl | Maize starch | Melt-processing | 15–30 | 160 °C, 2 min | 79 |
2 | [(C1C2)C1im]Cl | Corn starch | Mixing | 23 | 75 °C, 20 min | 215 |
3 | [C4C1im]Cl | Cellulose acetate | Micro-compounder | 20–40 | 150 °C, 150 rpm | 217 |
4 | [C4C1im]Cl | Starch, zein | Micro-compounder | 23 | 130 °C, 3 min | 80 |
5 | [C2C1im]Cl | Wood flour | Twin-screw extruder | 1–5 | 150–175 °C | 218 |
Ou et al.218 proposed that improving the thermoplasticity of rigid cell walls of lignocellulose using ILs may be a viable strategy to enhance the processability of wood based thermoplastic composites. They pretreated the wood flour with [(HO)2C2C1im]Cl to different weight percent gains and the pretreatment effect on the rheological properties of the resulting wood flour/high density polyethylene (HDPE) composites was investigated. It was observed that [(HO)2C2C1im]Cl treatment significantly reduced the crystallinity and improved the thermoplasticity of wood fibers and thus broadened the processing window of wood fiber/HDPE composites with stable flow and remarkably smooth product surfaces. It was further concluded that filler–filler interaction had a dominant effect on the extrusion/injection processing of WPCs and that IL pretreatment of wood fiber laid the groundwork for future research in WPCs. A recent report193 also describes the novel use of ILs in production of WPCs. Ball-milled mulberry wood (BMMW) was firstly kneaded in [C4C1im]Cl or [C4C1im][OAc] with the help of the cosolvent DMSO. Plasticization of BMMW for injection-moulding was achieved after the structural destruction of the wood cell walls in the presence of ILs (Fig. 19). The thermal flow and mechanical results revealed that the WPC obtained with DMSO/[C4C1im][OAc] exhibited better thermal and mechanical properties than those obtained with DMSO/[C4C1im]Cl, indicating that the thermoplasticity of AWP was related to the ILs’ anionic ability to disrupt the hydrogen bond networks in the rigid crystalline region of cellulose.
Fig. 19 Process flow diagram of the preparation AWP and appearance of mulberry wood before kneading (a), the kneader (b), mulberry wood after kneading (c), final AWP (d) and injection moulded AWP specimens (e).193 |
Recently, we have reported the pretreatment of lignocellulosic oil palm biomass with imidazolium based ILs.84 The oil palm biomass was ground to a particle size range of 250–300 μm and dissolved in [C2C1im][Et2PO4] or [C4C1im]Cl at 80 °C for 3 h and then regenerated using an acetone–water mixture as an anti-solvent. Afterward, the precipitated solid residue was separated by decantation and washed with distilled water 4–5 times to ensure complete removal of any residual IL. The anti-solvent acetone/water was removed from the supernatant using a rotary evaporator to leave an IL/lignin rich phase. The dried regenerated cellulose-rich fiber was compounded with a thermoplastic starch polymer and was thermally pressed at 160 °C for 10 min in a Carver Laboratory compression molding machine (CARVER, INC. USA) to fabricate a ‘green’ composite board (Fig. 20). The cellulose contents in the regenerated fibers after IL pretreatment were significantly increased, which resulted in a more accessible surface area for binder interaction and improvement in the thermal stability of the fibers. Due to removal of non-cellulosic impurities from the fiber surface as a result of the IL pretreatment, the composite panels fabricated from IL pretreated fiber exhibited superior mechanical and thermal properties. It was clearly indicated that IL-assisted pretreatment could be a new, highly promising and green technology for effective utilization of lignocellulosic biomass in the wood composites and related industries.
Fig. 20 Manufacturing steps to fabricate composite board from ILs treated oil palm fiber.84 |
Modeling of the process is essential to estimate the energy requirement and cost for dissolution processes, so that the comparison with other pretreatment methods will be possible. The first techno-economic analysis of IL based processing system was carried out and the following order of importance/sensitivity among the investigated variables was reported: IL price > biomass loading > recycling rate.243 Furthermore, molecular level simulation should be employed for better understanding the interaction mechanism of ILs with biobased molecules.244 Additionally, an optimized selection of both cations and anions as well as processing conditions may allow the design of IL systems with unique physicochemical characteristics that could lead to more effective utilization of ILs in lignocellulosic and biopolymer based composite industries.53
Despite all of these advantages and potential applications, ILs currently suffer from clear and significant disadvantages of their high cost that stand in the way of many commercial applications. It is believed that ILs normally fall in the range of 5–20 times more expensive than molecular solvents.245 The most common criticism of ILs that the authors encounter is that of the ‘severe’ limitations placed upon their large-scale deployment by their high cost. But are ILs inherently expensive, or is this opinion a consequence of the specific ILs that are historically prominent (dialkylimidazolium cations with polyfluorinated anions)? To answer this question, Welton and co-workers evaluated the economic feasibility and determined the cost price of two ILs synthesized from cheap raw materials.246 The cost prices of triethylammonium hydrogen sulfate and 1-methylimidazolium hydrogen sulfate were estimated as $1.24 kg−1 and $2.96–5.88 kg−1, respectively, which compares favourably with organic solvents such as acetone or ethyl acetate, which sell for $1.30–$1.40 kg−1. These results indicate that ionic liquids are not necessarily expensive, and therefore large-scale IL-based processes can become a commercial reality. In addition, organic electrolyte solutions containing a small fraction of ILs with potential dissolution capabilities for large amounts of cellulose can lead in the future to very interesting developments.247 Dissolution and depolymerization of cellulose in organic acid-catalyzed 30 wt% aqueous solution of NaCl under mild reaction conditions (100–125 °C) and some pressure (10–30 bar) has also been observed.248 It was proposed that such salt solutions disrupt the hydrogen-bond matrix among cellulose fibrils in a mechanism similar to that of ILs. Development of such ILs based solvent systems, in the future, might open new pathways in the field of biopolymer processing in which environmental concerns can be tackled, while simultaneously keeping process costs (energy, substrates, catalysts, etc.) at an acceptable level.
It must also be emphasized that conventional dissolution processes employ various solvent systems under extreme conditions and also are limited due to their dissolving capability, solvent recovery, toxicity, uncontrollable side reactions and high cost during biomass processing and derivatisation, the following list summarizes the major aspects for IL based processing of natural fibers and polymers that should receive attention for future research.24
■ Low cost of ILs and co-solvents (typically less than $2.50 kg−1)
■ Decreased IL loading
■ Short dissolution time
■ Applicability to a wide range of raw materials
■ End of use IL recovery
■ Optimized recycling to reduce ILs losses
■ Reduced influence of residual IL on downstream processing
■ ‘Greenness’ of ILs (environmental and health impact).
This journal is © The Royal Society of Chemistry 2017 |