Improved dispersion of cellulose microcrystals in polylactic acid (PLA) based composites applying surface acetylation
Graphical abstract
Introduction
Microcrystalline cellulose (MCC) fibre, as the name suggests, is commercially available and has gained significant interest in the last decade (Lin et al., 2011, Frone et al., 2011). Some of the unique features of MCC include its morphology, low density and mechanical strength (Huda et al., 2006). Polylactic acid (PLA) based cellulose composites have been much explored in the recent years as the key material to develop the next generation of light weight and high performance materials for a variety of defense, infrastructure and energy applications (Jonoobi et al., 2010, Peterson et al., 2007, Pei et al., 2010). The necessity to improve the dispersion of cellulose microcrystals (MCC) in biopolymer matrix like polylactic acid (PLA) has become increasingly important to enhance the mechanical properties of such composites and thereby the commercial viability of such products for biomedical as well as flexible packaging applications (Braun and Dogan, 2009).
Dispersion is the key challenge to prepare such composites (Braun and Dogan, 2009). The presence of hydroxyl groups on the surface of cellulose restricts its homogeneous dispersion in PLA, by initiating agglomeration or entanglement (Dubief et al., 1999). A possible strategy to overcome this challenge is surface modification, where the hydroxyl group is partially replaced by another functional group. In general, surface functionality of cellulose microcrystals as carried out from the literature review study, can be broadly categorized into three groups; (I) native surface chemistry of the particle as a result of their extraction like acid hydrolysis by sulfuric acid (Frone et al., 2011) (II) physical adsorption of surfactants or polyelectrolytes (Oksman et al., 2006) and (III) covalent modification such as esterification/etherification Braun and Dogan (2009), silylation (Pei et al., 2010) and polymer grafting (Pracella et al., 2010).
In this work the hydroxyl groups, as present on the surface of cellulose was partially substituted by acetyl groups by using acetyl chloride at room temperature. The primary motivation behind this study was to improve the dispersion of MCC in PLA matrix by surface acetylation. Emphasis was laid to characterize the behaviour of dispersion at different level of loadings, ranging from 1 to 5 wt%. Beyond 5 wt% loading, filler–filler interactions were observed. Composites were prepared with this surface acetylated MCC, using PLA as base matrix by solution casting technique and dichloromethane as the solvent (Mukherjee et al., 2012). Acetylation is confirmed by FTIR and NMR study. The behaviour of dispersion is characterized by XRD, DSC, and shear rheological tests. A rheological percolation threshold is calculated to quantify the level of dispersion and the optimal loading for a uniform dispersion.
Section snippets
Material
A polylactic acid biopolymer (Nature Work PLA Polymer 4032D) with a density of 1.24 g cm−3 and a melting point of 160 °C was chosen as matrix. A microcrystalline cellulose (MCC) with a mean particle size of 20 µm, supplied in powder form by Sigma Aldrich was used as a raw material. Acetic anhydride, pyridine, dichloromethane was purchased from Sigma Aldrich (Mukherjee et al., 2012).
Acetylation of MCC
Acetylation was performed with constant stirring in a 100 ml round bottom flask. A suspension of 2 g of MCC and 20 ml of
Acetylation of MCC
Acetylated cellulose was characterized by Fourier transform infrared (FT-IR) and solid-state cross-polarization magic angle spinning carbon-13 nuclear magnetic resonance (CP/MAS 13C-NMR) spectroscopy. The reaction scheme with acetyl chloride with FT-IR absorbance spectra of unmodified cellulose and acetylated cellulose resulting from the two reactions is illustrated in Fig. 1. For the pure MCC, as reported in literature, a strong band around 3, 434 cm−1 is observed which is attributed to the
Conclusions
Successful acetylation of microcrystalline cellulose (MCC) was accompanied by acetyl chloride at room temperature as revealed in NMR and FTIR studies. Improvement in dispersion was observed in morphological and rheological tests, when composites were prepared by this surface acetylation technique, particularly at a lower loading reaching its optimum value around 2.5 wt% from the rheological percolation threshold analysis, indicating that beyond this region, dispersion is affected by
Acknowledgments
The authors would like to acknowledge the facilities, and the scientific and technical assistance of Prof F. Seperovic for providing us the facility to conduct solid state NMR at Bio21 Molecular Science and Biotechnology Institute, The University of Melbourne, Mr Phil Francis and Mr Peter Rummel, of the Australian Microscopy and Microanalysis Research Facility at the RMIT Microscopy and Microanalysis Facility, RMIT University, Frank Antolosic Mike Allan and Muthu Panniselvam, from the
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