Recent developments in nanocellulose-based biodegradable polymers, thermoplastic polymers, and porous nanocomposites
Graphical abstract
Introduction
Nanocellulose is a new generation of nanomaterials that has been receiving extensive attention from scientists and industry because of its specific chemical and physical properties. Nanocellulose is an abundant and renewable nanomaterial that combines low density, high strength, and flexibility with chemical inertness and the ability to modify its surface chemistry. Nanocellulose is typically divided into two major groups based on their preparation techniques: cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs). CNCs can be obtained by acid hydrolysis, and sulfuric acid is the acid commonly used for producing sulfonated CNCs [1], although CNCs with other surface groups have been produced by hydrolysis with hydrochloric [2,3], phosphoric [4], hydrobromic [5], and phosphotungstic [6] acids. However, the production of CNFs often requires chemical pretreatment prior to mechanical disintegration processes such as homogenization, microfluidization, and super-grinding. The geometric dimensions and final properties of CNCs and CNFs are directly dependent on the cellulosic source, the preparation and processing conditions, and the possible post- or pretreatments. CNCs typically have a diameter of a few nanometers and their lengths range from 10 to 500 nm, whereas CNFs have diameters of 3–50 nm and lengths of a few micrometers. More information can be found in recent reviews [[7], [8], [9]].
A broad range of applications for both CNCs and CNFs are today being investigated, including their use in food packaging [10], coatings [11], biomedical applications [12], and printed electronic devices [13]. Among the various applications, nanocellulose-reinforced nanocomposites have been studied extensively because of their numerous advantages such as low cost, low density, renewable nature, low energy consumption, high specific properties, biodegradability, and the relatively reactive surface of the nanocellulose. Properties and some application of CNCs and CNFs are furnished in Table 1. It is worth noting that properties and geometry of CNCs and NCFs depend of the cellulose source and preparation technique. The use of CNCs or CNFs in various application is based on the required properties in the final product. Considering modulus and strength of CNCs and CNF certain nanocomposites with required stiffness or flexibility can be designed. Some of the challenges and drawbacks of using CNCs and CNFs are discussed in the concluding section.
Biodegradable polymers have become polymer matrices of interest in recent years because of the depletion of fossil resources and environmental concerns such as global warming. Therefore, there is an urgent need to develop renewable, source-based, environmentally benign polymeric materials (biopolymers), especially for use in short-term packaging and disposable applications. Such materials would not involve the use of toxic or noxious components in their manufacture, and they could allow for composting into naturally occurring degradation products [21].
The ideal biopolymer is of renewable biological origin and is biodegradable at the end of its life. Starch, cellulose, chitosan, and proteins are a few representatives of this ideal biopolymer. In addition, polycaprolactone (PCL) and poly(lactic acid) (PLA) are examples of polymers that have synthetic origin but are biodegradable. However, the substitution of biopolymers for commercial synthetic polymers has some serious challenges, such as brittleness, low thermal stability, and poor barrier properties [21]. Incorporation of nanocellulose into polymer matrices provides desired improvements in performance and properties, without affecting the biodegradability of the polymer.
Thermoplastics are the most commonly used non-biodegradable polymeric matrices. Thermoplastics are polymers that require heat to make them processable; after cooling, such materials retain their shape. In addition, these polymers can be reheated and reformed, often without significant changes in their properties. The introduction of nanocellulose into thermoplastics has two main challenges: processing time and compatibility between the nanocellulose and the matrix. Thermoplastics can be used to fabricate nanocellulose-reinforced composites only if the processing temperature, the temperature at which the fiber is incorporated into the thermoplastic polymer matrix, does not exceed 300–350 °C, depending on the type of nanocellulose. Because of the hydroxyl groups on the surface of the nanocellulose, the nanocellulose can be modified physically or chemically. The modification and functionalization of nanocellulose could enhance its thermal properties and increases its compatibility with nonpolar thermoplastic polymeric matrices.
The research interest in nanocellulose composite foams and aerogels is recent but rapidly growing.
An aerogel is a highly porous solid of ultra-low density with nanometric pore sizes formed by the replacement of the liquid in a gel with a gas. Foams, however, are defined as solid porous materials with micrometric pore sizes [22,23]. Inorganic and polymeric foams and aerogels are used in a wide range of applications because of their unique mechanical and acoustic properties, low density, and extraordinary low thermal conductivity. However, they are often extremely brittle. Incorporation of nanocellulose into the polymer foam or aerogels could significantly enhance their mechanical properties [24]. The combination of an ultra-low density, tunable porous architecture, and outstanding mechanical properties makes foams and aerogels of interest for a wide range of applications, including use in kinetic energy absorbers, biomedical scaffolds, thermal insulation, construction, separations and devices for the storage and generation of energy.
A number of review articles focusing on the use of nanocellulose- reinforced polymer composites have been published recently. Dufresne [25,26] focused on the various processing technique of nanocellulose based polymer composites, specially, advantages and challenges of wet and melt processing of both CNC and CNF based composites. Ng et al. [27] reviewed mechanical and thermal properties of cellulose-based nanocomposites. Oksman et al. [28] reviewed the recent development of the processing techniques of cellulose nanocomposites with focus on casting, melt processing and resin impregnation. Klemm et al. [29] described the production, structural details, physico-chemical properties of nanocellulose. Potential application of nanocellulose in the area of cosmetic products, wound dressing, drug carriers, medical implants, tissue engineering, food and composites have been also discussed in this review paper. Hubbe et al. [20] reported a review on nanocellulose in thin film, coating and plies for packaging applications. The aspect of this review was to evaluate the effect of nanocellulose on oxygen barrier and water vapor transmission performance, strength properties and the susceptibility of nanocellulose-based film and coating to the presence of humidity or moisture. Various promising strategies to prepare ecologically-friendly packaging materials based nanocellulose have been also discussed. Kargarzadeh et al. [30] addressed critical review on the manufacturing process of CNC and CNF composites. The review also provides advances on rubber and thermoset polymer matrices reinforced with CNCs and CNFs. The paper concludes with new findings and cutting-edge studies on electrospun nanocellulose composites. In another paper by Mondal [31] fabrication, properties and application of nanocellulose reinforced various polymer matrices including hydrophilic and hydrophobic matrices have been emphasized. De France et al. [32] highlighted the chemistry, preparation, properties, and applications of “nanocellulose-only” and “nanocellulose-containing” gels. Physical and chemical cross-linking strategies, post modification steps, and routes to control gel structure were also discussed, along with key developments and ongoing challenges in the field. Lavoine and Bergstrom [23] have also described preparation and properties of nanocellulose-based foam and aerogels focusing on their application in thermal insulation and energy storage. A comprehensive review of the recent advances made in the production, chemical and physical surface treatment procedures and life cycle assessment of CNFs and CNCs has been published by Kargarzadeh et al. [33]. Trache et al. [34] has also addressed the recent progress and challenges in the production methodologies of cellulose nanocrystals from various cellulose resources.
It can be observed that most prior publications focused on the use of either CNC or CNF and focus on one subject only. Certain recent advances and finding have not been addressed satisfactorily in previous publications, while here, we provide a comparative study on the effects of CNC versus CNF on the properties of composites incorporating them.
This review describes and discusses the processing, properties, and applications of nanocellulose-reinforced biodegradable polymers, thermoplastics, and foam and aerogel nanocomposites. Wide ranges of degradable or non-degradable polymers reinforced with nanocellulose have been classified and studied. All the aforementioned obstacles and challenges that need to be overcome to enable the production of these three classes of nanocomposites with tailored properties for novel applications are highlighted and investigated. As far as we know, no previous publication has reported such a critical review on these classes of nanocomposites.
Section snippets
Nanocellulose-reinforced biodegradable polymers
Nanocellulose-based composites have been studied extensively because of the promising reinforcing effect of these nanoparticles in numerous polymeric matrices. An improvement of the shear modulus by more than two orders of magnitude was observed in cellulose nanocrystal-reinforced poly(styrene-co-butyl acrylate) latex nanocomposites [35]. Since then, researchers have attempted to incorporate nanocellulose into various polymers, as shown in Fig. 1. Broadly, the polymeric matrices used for the
Nanocellulose-based thermoplastics
In contrast to crosslinking thermosets, whose cure reaction cannot be reversed, thermoplastics harden when cooled but retain their plasticity; that is, they can be remelted and reshaped by reheating them above their processing temperature without negatively affecting the material’s physical properties. A number of different types of thermoplastics have been used to produce cellulose nanocomposites. Nanocellulose enhances some of the properties of thermoplastic polymer matrices, which are
Porous nanocellulose composites
A foam material is formed by trapping many gaseous bubbles in a liquid or solid. It is normally an extremely complex system consisting of polydispersed gas bubbles separated by draining films [8]. In this section, composite foams in a solid state derived from the enhancement of nanocellulose and polymeric matrices are discussed. A polymeric foam is a solid cellular material formed by combining a solid polymer and a gas. The gas occupies the major volume of the foam, making it much lighter than
Challenges, outlook and concluding remarks
Nanocellulose is a new generation of nanomaterials that has become one of the most important nanomaterials from bioresources of the 21st century because of its superior physical and chemical properties, nontoxicity, renewability, biodegradability, etc. A number of methods have been reviewed that enable nanocellulose to be extracted from either plant or animal sources. Nanocellulose is typically characterized based on its preparation technique: cellulose nanocrystals (CNCs) are obtained by acid
Acknowledgments
The authors, Ishak Ahmad and Hanieh Kargarzadeh, would like to thank the Universiti Kebangsaan Malaysia (UKM) and Ministry of Higher Education of Malaysia (MOHE) for providing research grants, FRGS-MRSA/1/2016/STG07/UKM/01/1 and DIP-2016-026 respectively, and also of the National Science Centre, Poland on the basis of the decision number 2016/23/B/ST8/03509 to make this research possible.
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