Macromolecular NanotechnologyMelt processing of cellulose nanocrystal reinforced polycarbonate from a masterbatch process
Graphical abstract
Introduction
An intensive interest is paid to the use of nanoparticles extracted from biomass and renewable resources. Among them, cellulose nanomaterials or nanocellulose extracted from natural fibers using mechanical or chemical procedures are most probably the most promising materials. Many potential applications are envisaged for these astonishing nanomaterials but the most obvious one is related to their reinforcing capability in nanocomposite applications [1], [2], [3], [4], [5]. This makes sense given the structural function of cellulose in nature.
The effective utilization of nanofiller in nanocomposite materials strongly depends on their homogeneous dispersion within the polymer matrix to avoid the loss of the nanoscale and reduction of the specific surface area. To tackle this issue most investigations reported in literature used liquid medium and casting/evaporation as the processing technique benefiting from the good dispersion level of unmodified cellulose nanomaterials in water or polar liquids, or modified nanoparticles in apolar liquid medium. In view of the emerging marketing of nanocellulose, more industrial processing techniques are highly desirable. Melt processing techniques, such as extrusion and injection molding are obviously the targeted techniques.
In composite science the classical strategy consists in matching adequately the surface properties of the dispersed particles and continuous phase to improve the interfacial adhesion. Covalent and non-covalent functionalization and the use of surfactants are common tips to disperse nanocellulose in polymers [6]. Covalent functionalization is obviously the favorite strategy that allows chemists to freely express their expertise because of the reactivity of cellulose. However, covalent functionalization of the nanofiller normally involves complicate and expensive steps which can be prohibitive for most industrial uses. Moreover, it introduces defects and deteriorates nanocellulose percolation, and the full potential of the nanofiller is lost leading to mechanical properties which are far from the expectations, even if improvement is observed compared to direct extrusion of unmodified nanoparticles [7], [8].
Non-covalent functionalization is an easier way to prevent aggregation of the cellulosic nanofiller within the polymeric matrix. Poly(ethylene oxide) (PEO) has been used as a compatibilizing agent for the melt processing of cellulose nanocrystals reinforced polymer nanocomposites [9], [10]. After mixing the nanoparticles and PEO in water, the freeze-dried mixture was used to prepare nanocomposite materials with a low density polyethylene (LDPE) matrix by extrusion. Greatly improved dispersion of the cellulose nanomaterial was observed but unfortunately the mechanical properties of ensuing materials were very poor because only the intrinsic mechanical properties of cellulose nanocrystals were involved [10]. A similar approach was used to prepare CNC reinforced polylactic acid (PLA) nanocomposites [11]. A spray freeze drying technique was also shown to improve the dispersion of CNC in polypropylene (PP) over spray drying or freeze drying [12], [13].
An interesting approach recently reported for the preparation of CNC reinforced LDPE consisted in preparing first an aerogel by exchanging the solvent of an aqueous CNC dispersion against acetone, impregnating the resulting organogel, in which the CNCs form a percolating network with a hot LDPE solution in toluene, and compression-molding the resulting materials [14]. Even if this template process in its present form is not directly scalable for technological exploitation, the fact that the high level of dispersion was largely maintained upon compression-molding films and also reprocessing and “diluting” such nanocomposites in an extruder bodes well for the development of alternative mixing approaches. However, it was shown that mixer design and in particular the shear rate that is applied during processing may have a significant influence on the properties of polymeric nanocomposites reinforced with CNC [15].
In the present study, coating of cellulose nanocrystals (CNCs) was performed using the same polymer as for the matrix using a dissolution/precipitation process. The ensuing highly concentrated masterbatch was then diluted by extrusion. A similar strategy was used for processing CNC reinforced polyamide 6 nanocomposites but the CNC concentration was limited to 1 wt% [16]. Polycarbonate (PC) was chosen in the present study because it is a thermoplastic polymer that is easily worked, molded, and thermoformed. It is highly transparent to visible light, with better light transmission than many kinds of glass and because of this property PC finds many applications in optical devices. This polymer is a durable material, but although it has high impact-resistance, its scratch-resistance is poor and so a hard coating is generally applied to polycarbonate eyewear lenses and polycarbonate exterior automotive components. However, the melt processing of CNC reinforced PC is a major and ongoing challenge due its high viscosities and required high temperatures.
Section snippets
Materials
Polycarbonate (PC) used in this work was a commercial grade MAKROLON LQ2647, kindly provided by Bayer S.A.S. Pyridine was purchased from Sigma–Aldrich. Cellulose nanocrystals (CNCs) with 1.1% sulfur content were purchased from the University of Maine as an 11% aqueous suspension and used without further purification.
Solvent exchange
The main issue with nanocomposites is related to the poor dispersion of the dispersed nanomaterial in the continuous matrix. To overcome this obstacle and avoid self-aggregation of
Microscopic observation
Atomic force microscopy (AFM) images in Fig. 1a show the classical rod-like morphology of CNC sample. The average length and diameter were 365.0 ± 142.2 nm and 34.4 ± 7.65 nm, respectively. The presence of some bigger particles in the AFM images was also reported. These particles were not considered in the size calculation due their micrometric nature. The presence of these particles can be detrimental for the properties of the final composite, since they can have very different crystallinity and
Conclusions
The first part of this study aimed in preparing a masterbatch consisting of cellulose nanocrystals (CNC) and polycarbonate (PC). The CNC/PC masterbatch capsules were prepared using a dissolution/precipitation process in which water from the CNC aqueous dispersion was exchanged to pyridine serving as solvent for PC. The mixture was subsequently precipitated in water allowing the recovery of the masterbatch capsules. Improved thermal stability for PC-coated CNC masterbatch compared to pristine
Acknowledgments
The authors gratefully acknowledge “Ciência Sem Fronteiras” program for the financial support (PhD fellowship of M. M.) and M. Michel Bodenez (Bayer S.A.S., Puteaux, France) for providing polycarbonate granules. LGP2 and the Laboratoire Rhéologie et Procédés are part of the LabEx Tec 21 (Investissements d’Avenir – Grant agreement n°ANR-11-LABX-0030) and of the PolyNat Carnot Institut (Investissements d’Avenir - Grant agreement n°ANR-11-CARN-030-01).
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