Clean manufacturing of nanocellulose-reinforced hydrophobic flexible substrates
Graphical abstract
Introduction
The high abundance, strength and stiffness of NFC along with its biodegradability, low weight and sustainability have led to its being an attractive potential candidate for bio-composite applications (da Costa Correia et al., 2018). Nanoscale particles may substantially improve a polymer’s mechanical and dynamic properties even at low nanofiller content due to their high specific surface area and surface energy (Mandal and Chakrabarty, 2015). The addition of fillers is a well-established method to obtain high-performance polyolefins. However, the density of synthetic fillers is around 50–80% higher than nanoparticles derived from renewable resources (Hao et al., 2020). Thus, nanocellulose could potentially be used as a reinforcement material in lightweight nanocomposites based on polyolefins. In addition, the superiority of nanocellulose over synthetic fillers in terms of availability and environmental sustainability make it an appealing material for use in reinforcement purposes. The reinforcement effect of nanofibers of enhancing polymer performance has been a subject of interest in recent times (Zheng et al., 2019). The incorporation of nanocellulose into hydrophilic polymers such as poly (ethylene oxide) (Yong et al., 2018), poly vinyl alcohol (Sirviö et al., 2015), and epoxy (Abraham et al., 2016) has been successfully achieved, since those polymers have similar polarity and can be easily mixed in an aqueous solution. These nanocomposites can be employed in a wide range of biomedical, cosmetic and food applications (De France et al., 2017) in which water absorption is sometimes a desirable feature. However, drying NFCs while retaining their properties and the dispersion of NFCs in hydrophobic polymers as reinforcing agents remain challenges to the expansion of nanocomposite applications (Wang and Drzal, 2012).
The agglomeration of nanocellulose often occurs when polymers are filled with nanomaterials by means of melt compounding, leading to a considerable loss in mechanical properties. Only a few studies have been done on polyolefins and nanocellulose compatibilization techniques in order to minimize this issue. The lack of transfer efficiency of mechanical properties of nanocellulose from the single-fiber to the macroscale level such as hydrophobic polymers is still challenging due to poor dispersibility of nanofibrils into polymer matrices (Oksman et al., 2016). Melt compounding such as extrusion is the most employed method in polymer blend processing, but is rarely used for the preparation of NFC-reinforced polymer nanocomposites due to intrinsic incompatibility (Dufresne, 2017). To that end, chemical modification, solvent-exchange and freeze-drying have been used to incorporate nanocellulose into hydrophobic polymers. However, these methods impact the economics and logistics of material processing due to their labor-intensive and high-energy-demanding procedures (Beaumont et al., 2017). Freeze-dried NFC can be employed to remove water without it undergoing fiber agglomeration (Li et al., 2014). Zhu et al. (2017) obtained a Nylon/freeze-dried NFC filament with improved tenacity for textile applications. Recently, an intermediate product composed of polyethylene oxide (PEO) and nanocellulose was incorporated into polyethylene. The authors employed a solvent-free procedure; however, the freeze-drying method was applied to the cellulose nanocrystals (Inai et al., 2018). Surface modification of nanocellulose has been investigated in order to increase interaction between nanocellulose and polymer matrix and/or add new functionalities (Ferrer et al., 2017; Nie et al., 2021). After solvent exchanging from water to a nonpolar solvent, Yano et al. (2018) carried out chemical modification of NFC by linear and branched structures and melt extruded the product with polyethylene. The solvent exchange process of nanocellulose gradually removes the water content in a solvent by several centrifugations. Normally, the solvent exchange technique makes use of hazardous solvents such as dimethylformamide (DMF) (Bagheriasl et al., 2015), N-methyl-2-pyrrolidone (NMP) (Yano et al., 2018) and chloroform (Xu et al., 2016), and consumes high energy due to the centrifugations. CNC and NFC were subjected to solvent exchange procedures for the fabrication of polystyrene foam nanocomposites via the masterbatch and extrusion processes (Neves et al., 2019). Bagheriasl et al. (2015) obtained a stable suspension of freeze-dried CNCs and ethylene vinyl alcohol copolymer in DMF. Afterwards, the solvent-casted material was subjected to melt extrusion with polypropylene.
Only a few attempts have been carried out to avoid the use of harmful chemicals and high-energy treatments for nanocellulose processing. Therefore, better industrial nanocomposite processing techniques should be developed to make nanocellulose viable for industrial up-scaling (Dufresne, 2013). Interestingly, as observed by Orr and Shofner (2017), EVAL can be dissolved in a binary mixture of isopropanol (IPA) and water. At a certain volume ratio, EVAL becomes thermodynamically stable despite its considerable hydrophobicity. In this study, nanocomposites were prepared using a two-step process: a melt-mixing masterbatch process to disperse NFC in EVAL, followed by a melt extrusion process with HDPE (high-density polyethylene). Industrially, EVAL barrier films are fabricated through a coextrusion process with other polymer films to improve the barrier properties of the resulting packaging materials, such as EVAL/polyethylene and EVAL/polypropylene (Wu et al., 2016). HDPE and EVAL have natural interfacial adhesion due to their similar natures resulting from the presence of ethylene monomers. Obviously, the higher the ethylene content in the EVAL composition, the higher the interaction between HDPE and EVAL will be; in this study, the EVAL composition with the highest ethylene content in the plastic manufacturing industry was employed. Simultaneously, the presence of a hydroxyl group in the vinyl unit of EVAL may provide good interaction between nanofibers and ethylene vinyl alcohol copolymer chains.
Herein, a novel dispersion method was used, for the first time, to disperse the NFC in slurry form into EVAL followed by melt extrusion with HDPE. The aim of this study is to promote the development of cleaner production technology of bio-based and lightweight nanocomposites while making them more sustainable, economically viable and mechanically robust. Therefore, this approach may offer an excellent opportunity of meeting sustainable and mass-scale production of low-density and greener nanocomposites.
Section snippets
Material and methods
In this section we provide a detailed description of materials specification, the methodology used to prepare composites made from cellulose nanofibers and HDPE and their characterization methods.
Results and discussion
Results of mechanical, chemical, thermodynamic and structural properties of all composite films are analyzed and reported in this section and an improved theoretical understanding of their reinforcement effect is elucidated.
Conclusion
An economically attractive fabrication process free of harmful organic solvents was developed to achieve homogeneous distribution of NFC in polyolefin nanocomposites. The results show that the EVAL improved the interfacial miscibility which favored a better dispersion of NFC within the hydrophobic HDPE phase. The EVAL acted as a capping agent protecting the nanocellulose from irreversible agglomeration. The presence of NFC improved the mechanical properties, with a 22% and 98% increase in the
CRediT authorship contribution statement
Otavio Augusto Titton Dias: Conceptualization, Methodology, Software, Formal analysis, Investigation, Data curation, Writing - original draft, Writing - review & editing, Visualization, Project administration. Samir Konar: Methodology, Validation, Investigation, Data curation, Writing - review & editing, Project administration. Alcides Lopes Leão: Validation, Writing - review & editing, Supervision. Weimin Yang: Validation, Writing - review & editing. Jimi Tjong: Resources, Writing - review &
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The authors would like to thank the Ontario Research Fund-Research Excellence (ORF-RE), the Natural Sciences and Engineering Research Council of Canada (NSERC) and CNPq [Grant number 202275/2015–9] for the financial supports to carry out this research, and Onildo Dias Filho for the illustrations.
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