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Lomelí-Ramírez, M. G., Barrios-Guzmán, A. J., García-Enriquez, S., Rivera-Prado, J. J., and Manríquez-González, R. (2014). "Chemical and mechanical evaluation of bio-composites based on thermoplastic starch and wood particles prepared by thermal compression," BioRes. 9(2), 2960-2974.

Abstract

The present work inspects the preparation of bio-composites of cassava starch with particles of eucalyptus wood through the application of a novel method of thermal compression. Bio-composites with different amounts of wood particles (5 to 30%), with particle sizes of 4 and 8 mm, were obtained. Chemical and mechanical evaluation of these samples was carried out using optical microscopy, infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and the moisture absorption effect. The effect of the amount and size of the wood particles was tested by comparison with a thermoplastic matrix sample. Results from these evaluations demonstrated that the thermo-compression method produced bio-composites with a distribution of particles in the matrix that contributed to an increase in their tensile strength. This mechanical property is also enhanced by interfacial adhesion between the matrix and particles, as confirmed by SEM. Furthermore, the maximum amount of particles in the bio-composites (30%) showed the maximum resistance to moisture absorption. Temperature and time parameters contributed to the formation of diffraction patterns VH and EH as a consequence of the structural disruption of native starch. Finally, FTIR showed the chemical compatibility between the starch, glycerol, and wood particles.


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Chemical and Mechanical Evaluation of Bio-composites Based on Thermoplastic Starch and Wood Particles Prepared by Thermal Compression

María Guadalupe Lomelí-Ramírez,a,* Arturo Javier Barrios-Guzmán,b Salvador García-Enriquez,cJosé de Jesús Rivera-Prado,a and Ricardo Manríquez-González a

The present work inspects the preparation of bio-composites of cassava starch with particles of eucalyptus wood through the application of a novel method of thermal compression. Bio-composites with different amounts of wood particles (5 to 30%), with particle sizes of 4 and 8 mm, were obtained. Chemical and mechanical evaluation of these samples was carried out using optical microscopy, infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and the moisture absorption effect. The effect of the amount and size of the wood particles was tested by comparison with a thermoplastic matrix sample. Results from these evaluations demonstrated that the thermo-compression method produced bio-composites with a distribution of particles in the matrix that contributed to an increase in their tensile strength. This mechanical property is also enhanced by interfacial adhesion between the matrix and particles, as confirmed by SEM. Furthermore, the maximum amount of particles in the bio-composites (30%) showed the maximum resistance to moisture absorption. Temperature and time parameters contributed to the formation of diffraction patterns VH and EH as a consequence of the structural disruption of native starch. Finally, FTIR showed the chemical compatibility between the starch, glycerol, and wood particles.

Keywords: Bio-composites; Thermoplastic starch; Wood particles; Thermal compression

Contact information: a: Departamento de Madera, Celulosa y Papel, Centro Universitario de Ciencias Exactas e Ingeniarías, Universidad de Guadalajara, Km 15.5, Carretera Guadalajara-Nogales, Zapopan, Jalisco, México, CP. 45220; b: Departamento de Ingeniería Química, Centro Universitario de Ciencias Exactas e Ingenierías, Blvd. Marcelino García Barragán 1451, Guadalajara, Jalisco, México CP. 44430; c: Ingeniería-Básicas, Centro de Enseñanza Técnica Industrial, Nueva Escocia 1885, Guadalajara, Jalisco, México CP.44638; *Corresponding author: glomeli@dmcyp.cucei.udg.mx

INTRODUCTION

Starch is a natural polymer obtained from plants and composed chemically of amylose (20 to 30%) and amylopectin (70 to 80%) carbohydrates. These structures are formed by glucopyranose units connected with α(1-4) glycosidic bonds in a spiral chain and in linear and branched forms, respectively. Because starch is a natural polysaccharide with multiple botanical sources, it is considered to be a biodegradable, renewable, sustainable, and low-cost material. All of these benefits promote the potential use of this polysaccharide in papermaking additives, textiles, adhesives, thermoplastic starch (TPS) (Avérous and Halley 2009) for producing packing containers, eating utensils, and as raw material for film preparation (Vilaseca et al. 2007). The thermoplastic property is obtained when native starch is processed in the presence of a plasticizer at high temperatures and mechanical shear (Mo et al. 2011). This thermoplastic process is achieved when semi-crystalline starch becomes amorphous because of the substitution of the original hydrogen bonds between the amylose and amylopectine by the plasticizer (Prachayawarakorn et al. 2010). A plasticizer (e.g., glycerol, triethylene glycol, or oleic acid) with a high boiling point, in combination with starch, can be processed in a manner similar to a conventional synthetic thermoplastic (Corradini et al. 2006). However, this TPS has a lower mechanical strength in comparison with synthetic versions. Thus, to increase this mechanical property, natural fibers are added to reinforce the material. Some studies concerned with this issue have used cotton fibers (Prachayawarakorn et al. 2010), cellulose crystals from bamboo (Liu et al. 2008), and cellulose fibers from wood (Duanmu et al. 2010). The addition of natural fibers as a way to reinforce biodegradable polymers promotes the potential functionality and technological application due to their low cost, high availability, biodegradation, and considerably high resistance (Espinach et al. 2014). The amount of fibers used in the total weight of the thermoplastic is between 5 and 30%.

Because starch in the matrix (mixture with plasticizer) and fibers are obtained from natural sources, the resulting material is considered a bio-composite or “green” composite because it is completely biodegradable. Bio-composites are light materials and show good rigidity; however, the mechanical properties of the reinforced materials, such as the impact resistance, can be negatively affected when they absorb a certain amount of moisture (Kuciel and Liber-Knec 2009). Some approaches to overcome these problems have used chemical modification of the materials, mixtures of natural and non- biodegradable conventional polymers, as well as reinforcing the thermoplastics with inorganic materials and natural fibers (Corradini et al. 2006). The properties of composites reinforced with fibers depend on several factors, such as adhesion of the matrix and the amount and orientation of the fibers. Corradini et al. (2006) improved the matrix-fiber adhesion by chemically and mechanically modifying natural fibers used in composites.

Currently, the field of polymeric matrices reinforced with natural fibers is in constant development and is focused on composites prepared with biodegradable polymers and fibers, which furthers the objective of producing materials that are environmentally friendly (Vilaseca et al. 2007). Moreover, there is a growing interest in using these materials to replace plastic materials reinforced with synthetic fibers in a wide range of industrial applications, such as automotive, packing, and furniture production. In general, fibers based on cellulose, such as cotton, flax, hemp, henequen, jute, and timber, are commonly used for reinforcing plastics because of their high resistance, rigidity, and low density (Alemdar and Sain 2008). In light of this, preparation of TPS has been carried out by injection, extrusion, electrospinning, blow moulding, Haake torque rheometer, and polymer solution casting (Corradini et al. 2006). Another novel way to prepare TPS materials with direct thermoplastification by compression was recently developed and patented by Lomelí-Ramírez et al. (2011). In comparison with the traditional processes for moulding plastics described above, this new method offers several advantages: (1) the required equipment is cheaper, (2) the process of plasticization is achieved in one step, (3) the high viscosity of the thermoplastic starch is not a problem, and (4) the fibers are homogenously incorporated with the matrix as well as textiles (e.g., jute).

In the present work, this new methodology for preparing thermoplastic cassava starch, reinforced with different sizes and amounts of eucalyptus particles, is applied. The chemical and mechanical properties of the bio-composites were evaluated using optical microscopy, infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and the moisture absorption effect. From the results of this study, important information concerning the practical application and use of this methodology in similar bio-composites and new, biodegradable materials with desirable physicomechanical properties can be identified. Therefore, this process could be promising for diverse industrial purposes and from an environmental point of view.

EXPERIMENTAL

Material

The native cassava starch (Manihot esculenta) used in this study was donated by the J. A. Pasquini Company. Eucalyptus wood (Eucalyptus grandis) was used to produce the biocomposites. The commercial grade glycerol used as a plasticizer was donated by the Labsynth Company.

Preparation of Composites

Eucalyptus wood samples were milled and sieved to obtain particle sizes of 4 ± 1 and 8 ± 1 mm of length with width averages of 0.5 mm and 1 mm, respectively. Separately, cassava starch was mixed thoroughly with 30 wt.% glycerol in a plastic bag until attaining a homogeneous powdered mixture, which was used to prepare the bio-composites. Then, the wood particles (5, 10, 20, and 30 wt.%) were blended with this mixture by a 5 L mixer (Hobart model N50. São Paulo, Brazil). The prepared mixtures were poured into a stainless steel mold with a size of 170 mm × 170 mm × 3 mm, which was placed into a hydraulic press with an applied pressure of 40.25 MPa for about 50 min at 160 °C; a controlled cooling apparatus was also used to obtain the laminate TPS composites. A laminate matrix sample was also prepared as a reference.

Optical Microscopy

A Zeiss stereoscopic microscope (Discovery VI2, Jena, Germany) equipped with a Zeiss AxioCam camera was used to observe the transparency obtained in the composites and the distribution of eucalyptus wood particles in the composites.

Mechanical Testing

Tensile samples of the matrix and the composites were cut from respective laminates (plaques) using a laser cutting machine Gravograph LS100 (Säo Paulo, Brazil). The samples obtained were tested in a universal testing machine (EMIC DL 20.000 Paraná, Brazil) following the standard ASTM D 638M-96 (1998) procedure, at room temperature and a testing speed of 5 mm/min. Six to seven specimens were tested for each sample, and the average values of the tensile properties were recorded. All of the specimens were dried for 5 h at 60 °C in a laboratory oven and then kept in a desiccator prior to being tested.

Scanning Electron Microscopy (SEM)

A Philips XL-30 (North Billerica, MA) scanning electron microscope with an operating voltage of 10 to 20 kV was used to obtain SEM micrographs taken from fractured tensile test samples of both the matrix and composites. For this purpose, all samples (without special treatment) were coated with gold by sputtering in a vacuum chamber before observation.

X-ray Diffraction (XRD)

X-ray diffraction (XRD) studies were carried out using a Shimadzu diffractometer (Model XRD 7000; Kyoto Japan), with monochromatic Cu Kα radiation (k = 1.5418A⁰), at operation conditions of 40 keV and 20 mA, to determine the crystallinity of the materials. Samples were dried at 60 °C for 8 h, and the dimensions of the composite specimens used were 40 x 20 x 3 mm; the starch sample was measured in powdered form. The analysis was carried out in the 2θ angle range of 2.5° to 60° with a scanning speed of 1°/min.

Moisture Absorption

To assess the sensitivity of the bio-composites to moisture, humidity sorption isotherms of the composites were obtained. For this purpose, the samples were first dried in an air circulating oven at 60 °C. The samples were then conditioned at 0.004 wt% by keeping them in a desiccator at 22 to 25 °C and a relative humidity of 75 ± 2% using a saturated solution of sodium chloride (NaCl) that was prepared according to the ASTM E 104-2 (2007) standard. The samples were taken out daily to check the water absorption as a function of time until the weight became constant. The moisture content was calculated using the equation,

Moisture content (%) = [(M1M0)/ M0] 100, (1)

where M0 and M1 correspond to the mass of the samples (in g) before and after the oven thermal treatment at certain periods of time, respectively.

FTIR Analysis

FTIR spectrum analysis of the native starch was carried out using Bio-Rad FTIR equipment (Excalibur model FTS 3500GX; Cambridge, MA) with the potassium bromide disk technique. The FTIR analyses of the bio-composite samples were performed in a Bruker Vertex-70 (Bruker Optics, Germany) spectrophotometer using the attenuated total reflectance (ATR) technique. All IR spectra were then averaged out of 64 scans with a resolution of 4 cm-1 within a frequency range of 4000 to 700 cm-1.

RESULTS AND DISCUSSION

Optical Microscopy

In Fig. 1, the micrographs of bio-composites prepared with particle sizes of 8 ± 1 mm (a, b, and c) and 4 ± 1 mm (d, e, and f) are shown. The biocomposite samples observed in Fig. 1b and 1e correspond to thermoplastic material with a eucalyptus amount of 5%, and the samples shown in c and f were prepared with 20% eucalyptus particles. To analyze the thermoplastics in greater detail, enlargements of samples in Figs. 1b and 1e were made (Figs. 1a and 1d, respectively). As can be observed in all samples, an even distribution of wood particles in the thermoplastic starch matrix is clearly evident due to the obtained transparency. According to Canevarolo (2006), transparency of the thermoplastic is attained when the semi-crystalline starch is disrupted by plasticizer, temperature, mechanical shear, and pressure. Moreover, Fig. 1 shows the compatibility of the reinforcing material with the matrix and the organization of the wood particles in the bio-composites. Guimaráes et al. (2010) prepared composites of corn starch and banana fibers via thermo moulding. In the Guimaráes et al. (2010) study, the authors reported the formation of air bubbles during the moulding process that may have formed by the evaporation of water. In the present study, the absence of bubbles in the samples, which can produce defects in the composite, confirmed the success of the preparation and viability of the process. In addition, the methodology applied to prepare bio-composites in the present work avoids problems such as fluidity by high viscosity in the processing of the thermoplastic starches by extrusion and injection (Ma et al. 2005).

Fig. 1. Micrographs of bio-composites with eucalyptus particle sizes of 8 mm (a-c) and 4 mm (d-f) and amounts of 5% (b, e) and 20% (c, f). Micrographs (a) and (d) correspond to enlargements of samples (b) and (e), respectively.

Mechanical Testing

The results of mechanical testing of the tensile resistance of the bio-composites are shown in Table 1. Data regarding differences in strength and elongation were obtained with respect to the amount and sizes of particles. The tensile strength improved when the amount of wood particles increased, as can be observed in samples that contained 30% wood particles, as well as when the size of the particles was smaller. In contrast, the strain decreased with increasing amount and size of thermoplastic starch. This may be due to strong adhesion at the interface between the matrix and the smaller sized particles. Wicaksono et al. (2013) reported on the strengthening mechanisms associated with the ability of the fiber to form a network structure of the film matrix and attributed it to the high surface area contact of the smaller fibers. These results for the bio-composites are also corroborated with SEM micrographs (Fig. 2f). In addition, the high surface area of fibers promotes the formation of hydrogen bonds between matrix films and particles. Bilbao-Sainz (2011) found that enhancement of hydrogen bond formation can lead to higher efficiency in the stress transfer from the matrix to the fibers.

Table 1. Results of Tensile Properties of Matrix and Bio-composites

Numbers in parenthesis are standard deviations

Different letters, Significant at the 95.0% confidence level

It is well known that the geometry of a fiber, i.e., its morphology, architecture, orientation, arrangement, and volume fractions, influences the mechanical properties of composites. Of these factors, the volume fraction is one of the most important because many mechanical properties increase because of higher reinforcement loading; however a sufficiently high amount of the reinforcing material can also cause agglomeration and low dispersion of the fibers in the matrix (Fowler et al. 2006).

Ma et al. (2005) attributed the increase in strength properties to the fact that the matrix of TPS melts involving the fibers. Another factor was the intrinsic fiber-matrix adhesion caused by the chemical similarity between starch and the cellulose fiber. Also, the differences in performance of the composites suggest that the conditioning treatment used (60 °C for 5 h) in the present study was able to affect the function of the plasticizer, causing the starch chains to have better packing, resulting in better organization due to more intense interactions of macromolecules (amylase and amylopectin), which has been reported in previous studies (Lomelí et al. 2011). Chakraborty et al. (2007) showed the same effect by placing samples at 80 °C for 24 h and mentioned that the materials under these conditions showed better mechanical properties in relation to the composites conditioned at a relative humidity of 50%.

Guimarães et al. (2010) reported values ​​from 1.7 to 4.3 (MPa) for tensile strength, 24.6 to 484.0 (MPa) for elasticity modulus (MOE), and 1.20 to 75.56% elongation for corn starch bio-composites reinforced with various proportions of banana fiber and sugar cane bagasse fibers. Müller et al. (2009) reported values ​​between 1.59 and 26.6 (MPa) for tensile strength, 11 and 1047 (MPa) for modulus of elasticity, and 13 to 101% elongation for cassava starch films with various proportions of eucalyptus pulp fibers prepared by the casting method. The different values ​​reported in the literature depend on the type of starch (native or modified), the processing method (e.g., injection, molding, extrusion, casting), the amount, and the kind of reinforcement (e.g., fiber, pulp, wood). Thus, the correct choice of the components in the bio-composites can contribute to improving mechanical properties such as tensile strength and modulus of elasticity.

Scanning Electron Microscopy (SEM)

Figure 2 shows a series of SEM micrographs of the fractured surface of tensile tested specimens of composites containing different amounts of wood particles. Bio-composites with a eucalyptus particle size of 8 mm and with contents of 5 and 30% are depicted in Figs. 2a and 2b, respectively. In Fig. 2b, empty spaces produced from the mechanical detachment of the wood particles after the tensile stress essay can be observed, which implies a weak adherence between the particles and the matrix. Figure 2c shows, in detail, interfacial zones between the wood particles and matrix (indicated by arrows) of the 30% sample, where a small contact area between them can be seen. It is well known that the interface between the matrix and reinforcing material in composites is responsible for transferring the tensile stress from the matrix to the reinforcing material during mechanical strain. In other words, the tensile strength of these materials is attributed to the adherence between the components of the composites, as well as the intrinsic properties of the reinforcing material and polymer used (Kuciel and Liber-Knec 2009).

Fig. 2. SEM micrographs of fractured surface of bio-composites: (a) 5%, (b) 30%, and (c) detail of wood-matrix interface, particle size of 8 mm; (d) 5%, (e) 30%, and (f) detail of wood-matrix interface, particle size of 4 mm.

Figures 2d and e correspond to bio-composites prepared with a eucalyptus particle size of 4 mm and with a content of 5 and 30% as well. A smooth and homogeneous surface with evenly distributed wood particles in the matrix may be observed in these micrographs. Because of the small size of the particles (4 mm) in those bio-composites, it is possible to achieve a greater contact area (continuous phase) with the TPS. In this case, there were fewer empty spaces present after the tensile stress testing than there were with the bio-composite with a particle size of 8 mm. In Fig. 2c, it can be seen that the wood particles adhered well to the matrix, in accordance with the tensile strength data obtained for these samples (Table 1). This result suggests that the bio-composites with small particle sizes (4 mm) yielded better mechanical properties because they had a higher contact area than the 8-mm particles. According to Fowler et al. (2006), a reinforcing effect will be achieved when adherence between the reinforcing material and polymer matrix is good.

X-ray Diffraction (XRD)

In Fig. 3a, the XRD patterns of wood, native starch, and composites with a particle size of 8 mm are shown. Figure 3b corresponds to composites prepared with a particle size of 4 mm and a matrix sample. The native starch is partially crystalline (15 to 45%), and two polymorphic forms, A and B, were present. In addition, C, an intermediate polymorphic form, was also present (Ma et al. 2005). Cassava starch showed diffraction at angles of (2θ) 15.2°, 20.1°, 23.1°, and a doublet at 17.2° and 18.1°, which have been reported by Kawabata et al. (1984). These measurements corresponded to crystallinity type CA due to a 90% type A crystallinity and 10% type B crystallinity (Zobel 1988). Granular destruction, caused by the direct plasticization process in the press through the application of temperature, pressure, and a plasticizer, resulting in changes to the characteristic profile of the native starch (type-CA) in relation to type-EH and type-VH, as reported by Van Soest et al. (1996), can be observed in Fig. 3a and b. According to these authors, processing temperature, residence time, and cooling speed affect the final crystallographic pattern of starch-based materials. Accordingly, type-EHcrystallinity is promoted when residence time and temperature are increased. Therefore, in this study, the time and temperature used during processing may have contributed to the formation of Vand EHdiffraction patterns, as was previously reported by Lomelí-Ramírez et al. (2014). Also, Teixeira et al. (2009) reported that bio-composite samples display a diffraction peak around 2θ=16.8°, characteristic of amylopectin recrystallization, i.e., B-type crystallization. The diffraction peak corresponding to cellulose wood at 2θ=22.5° was clearly observed in the diffraction patterns of the bio-composites. As the amount of wood particles increased in the thermoplastic matrix, diffraction also increased (see Fig. 3a and b). This can be explained considering that cellulose I is the most common variety found in plants and is characterized by two well-defined peaks at around 2θ=22.5° and 2θ=12.5° (Panthapulakkal and Sain 2012).