Plant fibers have been studied as reinforcement in polymeric matrix in order to improve their physical and mechanical properties, as established practice for development of environmentally friendly products [
Starch is a promising material due to its high availability, renewable and biodegradable (even after being converted into a thermoplastic) character. Besides, this natural polymer presents interesting properties and characteristics for processing, being an attractive alternative to replace the synthetic polymers in applications that do not require long periods of use [
Some strategies have been tested in order to minimize the high hygroscopicity of the cellulose fibers and to improve the chemical compatibility between cellulose fibers and various polymeric matrices [
Hybrid organic-inorganic material was obtained in our previous work by deposition of SiO2 nanoparticles on cellulose fiber surface [
The inorganic precursor tetraethyl orthosilicate (C8H20O4Si–TEOS, 98%) for SiO2 synthesis was provided by Sigma Aldrich. Ammonium hydroxide (NH4OH–30% v·v−1) was the catalyst. Ethanol (CH3CH2OH–Neon 95%) was the solvent. Potassium sulfate (K2SO4–Vetec P.A.) was used for humidity control in the moisture adsorption test.
Cassava starch, composed of 85.5% amylopectin and 14.5% amylose (purchased from SM Ltda., Brazil); bidistilled glycerin plasticizer (Synth 98%); stearic acid (C18H36O2–Synth 98%); and anhydrous citric acid (C6H8O7–Chenco 98%) were used for preparation of the TPS extruded composites.
Unmodified cellulose fibers were kept in deionized water under mechanical stirring for 24 h in order to achieve total disintegration of cellulose sheets and proper fiber dispersion. Water to fiber consistency was 100 mL·g−1.
Fiber modification was carried out by sol-gel process based on previous studies [
After addition of TEOS, the reaction was kept under stirring for 18 h, under controlled environmental conditions (temperature of
The thermoplastic starch (TPS) was obtained from the physical mixture of cassava starch, glycerol, and deionized water in the mass proportions of 60/24/16, respectively. The contents of 1% (by weight) of stearic acid and 1% (by weight) of citric acid were used as antioxidants for extrusion, according to previous studies [
Morphological characteristics of the fibers and the fractured surface of the composites were evaluated by SEM micrographs in a JEOL JSM-6510 microscope, with a tungsten filament and operating at 15 kV. An energy dispersive spectroscopy (EDS) system (model JEOL 6742A–Ultradry Silicon Drift) with an active area of 10 mm2 and 132 eV resolution was used to detect and semiquantify SiO2 particles at the fiber surface. Average percentage of Si (% by mass) was obtained after five scans per sample in a 1
Starch (in powder), plain TPS, and composites were subject to thermogravimetric analysis (TGA) in a TA Instruments analyzer (model Q500) as proposed in Tonoli et al. [
Tensile tests of the TPS and composites were carried out according to ASTM D 638-10 [
Five 1.0 cm × 4.0 cm × 0.2 cm samples of each formulation were predried for 48 h at 60°C, weighed, and placed in hermetically closed containers with
Figure
SEM micrographs and EDS measurements (detail) of (a) unmodified and (b) modified cellulose fibers.
The deposition of SiO2 nanoparticles caused the coating of imperfections found in unmodified cellulose fibers. SiO2 deposition is achieved by the hydrolysis of the TEOS precursor and subsequent condensation of the resultant hydroxyl groups on the surface of the fibers [
The moisture adsorption pattern of the unmodified and modified cellulose fibers exposed to
Moisture adsorption of unmodified and modified fibers.
The thermal behaviors (TG and DTG) of the starch, TPS, and composites reinforced with modified fibers (MF) and unmodified (NMF) fibers are shown in Figure
(a) TG and (b) DTG of the starch (powder), TPS, and composites reinforced with unmodified (NMF) and modified (MF) cellulose fibers. Analysis ran under synthetic air atmosphere.
TPS exhibited a steady loss of weight from room temperature to near 250°C. This behavior is related to both the release of water adsorbed by the plastics during their acclimatization and combustion/volatilization of glycerol. This phenomenon hinders distinction between the first and second stage of TPS degradation and causes stronger mass loss ratio in the first stage in comparison to starch [
From the TG curve (Figure
Thermal properties (obtained by TG and DTG analyses) of the starch, TPS, and composites.
Sample |
|
DTG peak (°C) |
|
|
Residue at 600°C (%) |
---|---|---|---|---|---|
Starch | 300 ± 7 | 310 ± 3 | 321 ± 6 | 502 ± 2 | 0.4 ± 0.3 |
TPS | 291 ± 11 | 308 ± 1 | 321 ± 10 | 471 ± 4 | 0.4 ± 0.2 |
TPS + NMF (5%) | 290 ± 3 | 307 ± 3 | 320 ± 7 | 459 ± 2 | 0.4 ± 0.4 |
TPS + NMF (10%) | 291 ± 3 | 309 ± 2 | 325 ± 16 | 443 ± 2 | 0.0 ± 0.5 |
TPS + MF (5%) | 293 ± 4 | 310 ± 3 | 322 ± 6 | 475 ± 7 | 2.4 ± 2.2 |
TPS + MF (10%) | 292 ± 6 | 308 ± 3 | 320 ± 10 | 471 ± 2 | 3.7 ± 1.8 |
Figure
The stress to strain curves for tensile test of TPS and composites is shown in Figure
Stress to strain curves for tensile test of the TPS and composites reinforced with unmodified (NMF) and modified (MF) fibers.
Inclusion of cellulose fibers and increase in fiber load led to an increase of Young modulus (
(a) Young modulus (
Addition of 5% of unmodified fibers to TPS increased in 1335% and 433% Young modulus and tensile strength, respectively. Inclusion of 10% of unmodified fibers to TPS increased in 3329% and 633% Young modulus and tensile strength, respectively. The improvement of the mechanical properties (tensile) with inclusion of different plant fibers and nanofibers in TPS composites was widely reported in literature [
Scanning electron microscopy (SEM) was used to observe the interaction and dispersion of the cellulose fibers in the TPS matrix. Micrographs of the starch powder and fractured surface of TPS and composites are depicted in Figure
SEM micrographs of (a) raw starch; fractured surface of the (b) TPS; (c, d) composites reinforced with 5% and 10% of unmodified fibers (NMF), respectively; and (e, f) composites reinforced with 5% and 10% of modified fibers (MF). Arrows indicate holes in the composite after rupture.
The fractured surface of the TPS samples (Figure
The TPS adsorbs moisture due to its hydrophilic nature [
Moisture adsorption of the TPS and composites reinforced with unmodified (NMF) and modified (MF) fibers.
The addition of cellulose fibers contributed to decreasing of moisture adsorption of TPS (Figure
Lower enhancement of moisture adsorption found for composites reinforced with MF is possibly due to formation of microcracks at the interface between phases (arrow in the detail of Figure
Main results found in our work show that deposition of SiO2 nanoparticles hindered the interfacial interaction between the cellulosic fibers and the TPS matrix. Future investigations with nonpolar matrices are recommended to advance the development of composites reinforced with the hybrid SiO2-cellulose materials proposed in this work, since a remarkable decrease in hydrophilic nature of cellulose fibers was achieved.
The main proposition of this work was to develop a new high strength composite made from TPS and
The authors declare that there is no conflict of interests regarding the publication of this paper.
Financial support for this research project was provided by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Fundação de Amparo à Pesquisa do Estado de São Paulo (Fapesp), Fundação de Amparo à Pesquisa do Estado de Minas Gerais (Fapemig), Coordenação de Aperfeiçoamento de Pessoa de Nível Superior (Capes, Edital Capes-Embrapa), and Empresa Brasileira de Pesquisa Agropecuária (Embrapa), in Brazil. Thanks also go to Rede Brasileira de Compósitos e Nanocompósitos Lignocelulósicos (Religar).