Abstract
The tradition of using naturally occurring plant fibers is still alive in Africa. In the Uíge province of northern Angola, bast fibers from Triumfetta cordifolia serve as the basis for everyday objects, such as baskets, mats, fishing nets, and traditional clothing. The fibers exhibit a Young’s modulus of 53.4 GPa and average tensile strength of 916.3 MPa, which are comparable to those of commercial kenaf fibers. These values indicate a high potential for use as a reinforcement in biocomposites. Based on this promising mechanical and physical profile of individual fibers, different biocomposites were produced with polylactide (PLA) as a matrix. The obtained composites were analyzed mechanically, physically, and visually. Unidirectionally arranged PLA/33% T. cordifolia composites with continuous fibers showed the highest Young’s modulus (10.79 GPa ± 1.52 GPa) and tensile strength (79.37 MPa ± 14.01 MPa). These composites were comparable to those of PLA/30% hemp composites (10.9 GPa and 82.9 MPa, respectively) and therefore have economic potential.
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Triumfetta cordifolia: A Valuable (African) Source for Biocomposites
Peter Grosser,a Carolin Siegel,b Christoph Neinhuis,a and Thea Lautenschläger a
The tradition of using naturally occurring plant fibers is still alive in Africa. In the Uíge province of northern Angola, bast fibers from Triumfetta cordifolia serve as the basis for everyday objects, such as baskets, mats, fishing nets, and traditional clothing. The fibers exhibit a Young’s modulus of 53.4 GPa and average tensile strength of 916.3 MPa, which are comparable to those of commercial kenaf fibers. These values indicate a high potential for use as a reinforcement in biocomposites. Based on this promising mechanical and physical profile of individual fibers, different biocomposites were produced with polylactide (PLA) as a matrix. The obtained composites were analyzed mechanically, physically, and visually. Unidirectionally arranged PLA/33% T. cordifolia composites with continuous fibers showed the highest Young’s modulus (10.79 GPa ± 1.52 GPa) and tensile strength (79.37 MPa ± 14.01 MPa). These composites were comparable to those of PLA/30% hemp composites (10.9 GPa and 82.9 MPa, respectively) and therefore have economic potential.
Keywords: Bast fibers; Biocomposites; Lightweight; Tensile tests; Young’s modulus
Contact information: a: Institute for Botany, Technische Universität Dresden, Zellescher Weg 20b, 01217 Dresden, Germany; b: Institute for Natural Materials Technology, Technische Universität Dresden, Marschnerstrasse 39, 01307 Dresden, Germany; *Corresponding author: peter.grosser@tu-dresden.de
INTRODUCTION
In recent years, problems caused by the excessive consumption of fossil fuels have become increasingly visible. This development has triggered the formation of numerous new markets based on renewable resources. Established industries have also seen the potential of these new markets and have started to use bio-based materials. For example, in the automotive sector, natural fibers are now used as reinforcement material in various interior components, such as parcel shelves and interior door panels (Carus and Partanen 2017). Additionally, an entire new sector for natural fiber-reinforced plastics has been established and is steadily growing. These developments have led to an increasing demand for commercial fiber plants and renewed the interest in fiber plants that were once used commercially, but have fallen into disuse. The tropical plant Triumfetta cordifolia A. Rich can be regarded as an example of such a plant.
Triumfetta cordifolia is a fast-growing shrub and is widespread in moist areas of tropical Africa. Most local communities in these regions know the plant for its versatile medicinal properties or use it as food (Brink and Achigan-Dako 2012). Only in a few areas is T. cordifolia known to produce strong bast fibers, which also have an industrial history. In the 1950s, Belgium imported large amounts of these fibers from the DR Congo and used them to produce coffee bags (Brink and Achigan-Dako 2012). However, the fibers are rarely used today. In the Uíge province in northern Angola, people still use Triumfetta traditionally to produce ropes, textiles, mats, baskets, and other products (Senwitz et al. 2016).
In tensile tests, T. cordifolia fibers show an average Young’s modulus of 53.4 GPa, which is approximately twice that of flax (27.6 GPa) and jute fibers (26.5 GPa) and comparable to that of kenaf fiber (53 GPa). The tensile strength of T. cordifolia fibers (916.3 MPa) is similar to that of kenaf fibers (930 MPa) (Faruk et al. 2012; Senwitz et al. 2016). Moreover, T. cordifoliafibers show a density of 1.26 g/cm3, comparable to those of flax (1.5 g/cm3), hemp (1.48 g/cm3), kenaf (1.45 g/cm3), and jute fibers (1.3 g/cm3), which indicates the high potential of these bast fibers for use as reinforcement in biocomposites (Faruk et al. 2012; Mansor et al. 2013; Senwitz et al. 2016).
The first attempts to compound entire fiber bundles in a thermoplastic polylactide (PLA) matrix (NatureWorks 2004) led to poor results and had a Young’s modulus below 3 GPa (Petit 2016). However, because the mechanical and physical properties of the fiber are promising, a new approach was chosen, which produced long fiber-reinforced thermoplastics (LFRTs), as well as continuous fiber-reinforced thermoplastics (CFRTs). The results were compared to further evaluate the potential of T. cordifolia bast fiber as a reinforcing component.
EXPERIMENTAL
Materials
Fibers
The fibers originated from T. cordifolia plants growing in the province of Uíge (municipalities Bembe and Uíge) in northern Angola. The bast fibers were isolated from harvested T. cordifolia stems by peeling the whole bark, including the fibers, off of the wood. The fibers were subsequently separated from the bark using a knife (Senwitz et al. 2016). To ease the extraction of the fibers from adjacent cells, the samples were immersed for three weeks in plastic buckets containing tap water, which is a process called retting. This process microbially degrades non-cellulosic material (e.g., pectins) and helps to separate single bast fibers from adjacent cell material (Tahir et al. 2011). It has been previously reported that retting has positive effects on the mechanical characteristics of T. cordifolia fibers (Senwitz 2015).
Fibers from two different harvesting dates were used in this study. The first harvest took place in November 2016, at the beginning of the rainy season in this region. The second harvest took place in February 2017, during the middle of the rainy season. The fibers harvested in November (Figs. 1A and 1I) are referred to as ‘N-fibers’ and those in February (Figs. 1E and 1L) as ‘F-fibers’.
Matrix
The biodegradable polymer PLA (Indigeo Biopolymer 2003D, NatureWorks, Minnetonka, MN, USA) was used as the matrix material to produce the biodegradable composites.
Methods
Preprocessing
Because the use of whole fiber bundles as reinforcements in thermoplastics have been found to have negative side effects (pull-outs, inhomogeneous distribution, fractures, etc.), the bundles were separated (Petit 2016). Therefore, a close-meshed comb was used to isolate the individual fibers.
To compare several types of composites, two different fiber lengths were used. One half of the material was cut to a length of 0.5 cm to 1 cm (long fibers, Figs. 1B and 1F), the second half was left uncut after the combing process and included fibers of
10 cm to 15 cm length (continuous fibers, Figs. 1J and 1M).
The long fibers had to be brought into an appropriate form to be used in the film-stacking process with the PLA foil. Therefore, the material was transformed into nonwoven fabrics via the wet-laid method (DIN EN ISO 5269/2 2005), using a sheet former (System Rapid-Köthen, Frank-PTI, Birkenau, Germany). This process includes the swirling up of the fiber material together with water, before the whole mixture gets withdrawn through a screen. The resulting nonwoven fabrics, approximately 20 cm in diameter and 1 mm in thickness (Figs. 1C and 1G), were then vacuum dried.
The continuous fibers were parallelized and straightened using a flat iron. The unidirectionally (UD)-arranged continuous fibers (Figs. 1J and 1M) were fixed on the PLA foil for easier handling in the following processes.
All of the fiber materials were dried overnight in a drying oven at 105 °C. This step evaporated residual moisture and thus avoided the formation of evaporation airlocks during the following plasticizing process in the heating press.
Film-stacking
To embed the fiber material, the film-stacking method was used. This method can be used for short and long fibers, as well as continuous fibers, unlike injection molding and the extrusion method. The fiber material and PLA film were alternately layered.
Each LFRT consisted of three nonwoven fabrics (approximately 1-mm thickness) embedded into ten PLA foils (100-μm thickness). The fiber content in the LFRTs was 40% (dry matter content). For the CFRTs, two layers of UD-arranged continuous fibers (approximately 1-mm thickness) were embedded into eight PLA foil layers. The fiber content in the CFRTs was 33%. The compounds were then plasticized in a hot press (WPM Werkstoffprüfmaschine Leipzig DP 1500, WPM Leipzig GmbH, Markkleeberg, Germany) at a temperature of 190 °C and pressures of 5 kN and 15 kN for the LFRTs and CFRTs, respectively. Subsequently the composites were cooled down between iron plates. The results were circular nonwoven LFRTs (diameter = 20 cm) and square UD-CFRTs (10 cm × 10 cm), as shown in Fig. 1 (LFRTs: Figs. 1D and 1H; CFRTs: Figs. 1K and 1N).
From the available materials, five LFRTs were manufactured, three of which contained N-fibers and two contained F-fibers. Eight out of the 14 CFRTs contained N-fibers and six contained F-fibers.
Tensile tests
All of the composites were mechanically characterized and evaluated by determining the Young’s modulus, tensile strength, and breaking strain via static tension tests in accordance with DIN EN ISO 527-1 (2012). Complying with this standard, specimens were cut from the composites using a laser cutter (Legend EXT, EPILOG, Houten, The Netherlands).