Cellulose acetate and short curauá fibers biocomposites prepared by large scale processing: Reinforcing and thermal insulating properties
Introduction
Composites are multicomponent and multiphase materials that combine the properties of their constituents in different ways. In general, the properties of a given component is preserved or enhanced in the composites. Thus the properties of composites are determined by the properties of their components and mainly by interfacial adhesion and size, shape, orientation and distribution of the fillers in the matrix (Panigrahi et al., 2007). Polymer matrix composites (PMC) represent one of the most important classes of composites. Among these, the polymer matrix composites reinforced with fibers are technologically important because of the high mechanical strength of the fibers compared with the matrix (Joshi et al., 2004, Ragoubi et al., 2010). Usually polymer composites produced by continuous fiber pultrusion show better mechanical properties than those processed with short fibers (short fiber reinforced composite – SFRC). However, SFRC are more versatile with respect to preparation methods (Joshi et al., 2004, Mano et al., 2010).
Traditionally, polymer matrix composites are based on thermoset resins such as unsaturated polyester and epoxy or on polyolefins and synthetic fibers, for example glass or carbon fibers. However, polymer composites (Joshi et al., 2004, Barkoula et al., 2010) and nanocomposites (Abdul Khalil et al., 2012) based on natural fibers have been subject of interest for many researchers in the academy and in the industry. Most of the recent research with PMC is related to composites of polyolefins and lignocellulosic fibers like bamboo (Phuong et al., 2010, Dagang et al., 2012), curauá (Mano et al., 2010, Castro et al., 2012), banana (Annie et al., 2008), abaca (Rahman et al., 2009) and others. These papers often show improvement of the interfacial properties by means of surface treatments of the fibers or by using coupling agents. For example, Rahman et al. reported that mercerization decreases the hydrophilicity of abaca fibers, improving their adhesion with polypropylene (Rahman et al., 2009). Annie et al. showed the role of interfacial adhesion on the properties of composites by reporting higher thermal conductivity of the composites of poly(propylene-g-maleic anhydride) and short banana fibers in comparison with composites of polypropylene and the same fiber (Annie et al., 2008). In this case, the anhydride groups are responsible for interfacial adhesion resulting from chemical reactions with the hydroxyls on the surface of the fibers. In the last decade, composites based on biodegradable polymers and fillers from renewable sources, known as biocomposites (Mohanty et al., 2000), have received special attention. A series of papers on biocomposites has been published in recent years, for example biocomposites of poly(hydroxybutyrate) (PHB) and bamboo fibers (Krishnaprasad et al., 2009), poly(lactide); PHB and flax fibers (Arias et al., 2013); poly(lactic acid) (PLA) and continuous fibers of kenaf (Nishino et al., 2003); PLA and jute (Plackett et al., 2003); cellulose acetate and hemp (Mohanty et al., 2004); cellulose acetate and cellulose nanowiskers (Zhen-Yu et al., 2013) and polyester and curauá fibers (Harnnecker et al., 2012).
Cellulose acetate (CA) represents an important class of thermoplastic derived from cellulose. Cellulose acetate with different degrees of acetylation are amorphous, biodegradable and nontoxic, presenting many applications as filters, membranes, films, food packaging and capsules for drug delivery (Chandra and Rustgi, 1998). This class of polymers has been used as matrices for biocomposites with lignocellulosic fibers such as hemp (Mohanty et al., 2004) and flax (Taha and Ziegmann, 2006). An advantage of these composites over the composites based on polyolefins is the natural interfacial adhesion resulting from interactions between hydroxyls and carbonyls of the lignocellulosic fibers and of the matrix (Taha and Ziegmann, 2006).
Curauá fibers are extracted from the leaves of curauá, a plant originally from the Amazon. These fibers have been used in the automotive and textile industry due to their high Young's modulus in comparison with other lignocellulosic fibers (Mohanty et al., 2004). This makes curauá fibers promising for replacing synthetic fibers in composites (Castro et al., 2012, Zhen-Yu et al., 2013). Recently, we reported the mechanical and thermal properties of biocomposites based on cellulose acetate, curauá fibers and biodegradable plasticizers prepared using a bench scale extruder (15 cm3 capacity). These composites showed lower thermal conductivity and higher heat capacity in comparison with the matrix (Gutiérrez et al., 2012a, Gutiérrez et al., 2012b). Moreover, they combine typical mechanical properties of dense materials with thermal insulating characteristics of porous materials such as polyurethane or polystyrene foam and because of this they present a real potential of applications. The present work has the purpose of scaling up the processing of these biocomposites based on cellulose acetate, plasticizer and short curauá fibers, in order to develop a complete and functional formulation with a wide potential of applications that could be produced industrially.
Section snippets
Materials
Two commercial grades of cellulose acetates containing 20 wt% and 30 wt% of dioctyl phthalate (DOP) as plasticizer (Tenite Acetate 105, 39.8% acetyl and Mn = 30,000 g mol−1, supplied from Eastman Chemical Company, USA) were used to prepare the biocomposites.
Curauá fibers were obtained from EMBRAPA-PA, milled (average length (5 ± 1) mm) and dried (F). A portion of the curauá fibers was subjected to extraction with acetone for 8 h in a Soxhlet apparatus, in order to remove any waxes, and air-dried at 100 °C
Morphology of the biocomposites
The SEM micrographs of the surfaces of the fractures resulting from impact resistance test of the biocomposites show fibers with diameters ca. 100 μm (Fig. 1a, b and c) as well as fibrils with approximately 5 μm diameter (Fig. 1d, e and f), distributed throughout the matrix. Therefore, a partial fibrillation occurred and this is possibly due to the flow and shear imposed by the extrusion and injection processes and to the chemical treatment of the fibers This result is different from that
Conclusions
Biocomposites based on cellulose acetate and short curauá fibers can be prepared on a large scale by extrusion and their mechanical and thermal properties are useful for many applications. The mechanical properties can be tailored by controlling the composition of the biocomposites as well as by chemical treatment of the fibers. The chemical treatments of the fibers result in changes in the fiber–polymer interface and consequently improve the stiffness and increase the thermal conductivity and
Acknowledgments
M. Chavez G. acknowledges a fellowship from CAPES – program PEC-PG. The authors acknowledge Prof. C.H. Collins for manuscript revision, FAPESP (2010/02098-0 and 2010/17804-7) for financial support and EMBRAPA-PA for donation of the fibers.
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