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Exploitation of by-products from cassava and ahipa starch extraction as filler of thermoplastic corn starch
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Exploitation of by-products from cassava and ahipa starch extraction as filler of thermoplastic corn starch �pez b, M. Alejandra García a, d Versino Florencia a, c, *, Olivia V. Lo a
Centro de Investigaci� on y Desarrollo en Criotecnología de Alimentos, CIDCA (UNLP-CONICET-CICPBA), Facultad de Ciencias Exactas, UNLP, 47 y 116, (1900), La Plata, Argentina b Planta Piloto de Ingeniería Química, PLAPIQUI (UNS-CONICET), Departamento de Ingeniería Química, UNS, Camino La Carrindanga km. 7, (8000), Bahía Blanca, Argentina c Departamento de Ingeniería Química, Facultad de Ingeniería, Universidad Nacional de La Plata (UNLP), 48 y 115, La Plata, Buenos Aires, 1900, Argentina d Departamento de Química, Facultad de Ciencias Exactas, Universidad Nacional de La Plata (UNLP), 47 y 115, La Plata, Buenos Aires, 1900, Argentina
A R T I C L E I N F O
A B S T R A C T
Keywords: Biocomposites Sustainable materials Biodegradable Agroindustry by-products
Advances in bio-based composite materials offer potential opportunities for economic revalorization of agro industrial residues such as starchy tubers peels and bagasse. In this work, cassava and ahipa peels and bagasse are studied as potential fillers of thermoplastic corn starch (TPS) films. It was assessed the effect of studied organic fillers on thermal stability as well as optical, water vapor barrier, and mechanical properties of TPS. In all cases, type (peel or bagasse) and content (0.5 and 1.5 wt %) of the filler had a great impact on the biocomposites properties. Results demonstrated that these fillers modify TPS properties and gives utility to these residues with low economic cost.
1. Introduction Environmental problems associated with synthetic plastics and the exhaustion of fossil resources have led to a great interest in sustainable and ecologically friendly materials [1]. Biomass conversion into valu able materials like bio-based polymers and biocomposites has both substantial economic and environmental relevance [2]. Throughout the last decades numerous developments on biocomposites have been car ried out making them interesting alternatives for conventional materials in a vast number of end-uses. Recently, natural fibers are increasingly being used as reinforcing materials in polymers and composites. In comparison to inorganic fillers, lignocellulosic fillers are more attractive owing to their renewable nature, biodegradability, low energy con sumption, wide availability, low cost and density, and high specific strength and modulus [3,4]. Agroindustrial or agricultural residues can be considered important for natural fibers obtention [5]. Several authors have reported the use of natural fibers as fillers of starch biocomposites [6,7]. Particularly, the addition of peels and bagasse in starch compos ites development has also been reported by Teixeira et al. [8], Versino et al. [9], and Pasquini et al. [10]. Moreover, root and tuber crops are important to agriculture, food security, and income for 2.2 billion people in developing countries [11].
Among these crops, cassava (Manihot esculenta) is ranked as the fifth most widely used starch source in the world, and third among the food sources consumed in tropical regions [5]. Cassava is a robust crop that grows well in poor soils and in areas with low or unpredictable rainfall. However, to longer sustain high yields, it is important to prevent soil nutrient depletion and erosion, which can be achieved through simple agronomic or soil conservation practices such as intercropping [12–14]. In this regard, legumes such as Pachyrhizus ahipa, also called ahipa or Andean yam bean, are considered good alternatives to be intercropped with cassava since they improve soil fertility [15–20]. Both, cassava and ahipa tubers, accumulate starch in large quantities, hence representing a highly energetic food source. Starch production processes from roots and tubers is energy as well as water intensive and generates wastewater and solid waste (peels and bagasse) with high organic load which leads to significant water as well as air pollution [21,22]. Pandey et al. [23] reported that processing of 250–300 ton of cassava tubers results in about 1.6 ton of solid peels and about 280 ton of bagasse with a high moisture content (85%). Solid wastes are generally discarded in the environment in landfills without any treatment. Besides being used as animal feed or burned for energy purposes, no other practical applica tions have been found for these residues [24]. Therefore, there is an increasing interest in finding new options to produce value-added
* Corresponding author. E-mail address: [email protected] (V. Florencia). https://doi.org/10.1016/j.compositesb.2019.107653 Received 20 July 2019; Received in revised form 25 October 2019; Accepted 28 November 2019 Available online 29 November 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Versino Florencia, Composites Part B, https://doi.org/10.1016/j.compositesb.2019.107653
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products from these solid by-products. Accordingly, the combination of environmental awareness, economic drivers, and international policies encourage the recuperation and reutilization of agricultural and forest industry waste components [25]. A possible exploitation of peels and bagasse obtained from cassava and ahipa tubers is the use of these by-products as fillers of polymeric matrices, especially those based on bio-based and biodegradable polymers. Consequently, the aim of this work was to give added value to byproducts (peels and bagasse) derived from cassava and ahipa starch extraction by using them as fillers of thermoplastic corn starch films. Processing techniques employed to obtain these biomaterials were meltmixing and thermocompression. The effect of the presence of these organic fillers on the thermal plasticization of corn starch, as well as their influence on microstructure and final properties of the starch biodegradable films were evaluated.
maximum torque and plasticization energy were determined as it was �rdoba et al. [28]. proposed by Co
2. Materials and methods
Homogeneity and appearance of films were examined by Scanning Electron Microscopy (SEM), using a JEOL JSM-35 CF electron micro scope (Japan), with a secondary electron detector. Films were cryofractured by immersion in liquid nitrogen, mounted on bronze stubs and coated with a gold layer, using an argon plasma metallizer (sputter coater PELCO 91000).
2.3. Films preparation Thermoplastic starch films were obtained by thermo-compression using a hydraulic press, following the processing conditions reported in a previous work [29]. Mixtures were conditioned at 25 � C and 60% relative humidity (RH) previous to thermo-compression (140 � C, 150 kg cm 2, 6 min). Before characterization, films were conditioned again at 25 � C and 60% RH. Film thickness was measured at least in ten different locations with a digital coating thickness gauge CM-8822 (SolTec, Argentina). 2.4. Microstructural characterization
�n, Native corn starch was provided by Misky–Arcor (Tucuma Argentina) with an amylose content of 23.9 � 0.7% [26]. Cassava and ahipa peels and bagasse were obtained at laboratory scale using an �pez et al. [18] optimized method for starch extraction proposed by Lo from tubers of plants cultivated at INTA Montecarlo farm (Misiones, Argentina). These residues, named as cassava peel (CP), cassava bagasse (CB), ahipa peel (AP), and ahipa bagasse (AB), were dried at 50 � C to constant weight, ground, and sieved (through a 500 μm standardized mesh). Residues yield was calculated gravimetrically and expressed as g of dry by-product per 100 g of roots (wt.%) and reported values corre spond to the mean of three determinations for each residue. They were chemically characterized following standard methods as reported in previous works [9,27]. In brief, quantification of moisture and total ash content were determined gravimetrically at 105 � C until constant weight and according to AOAC 923.03 method, respectively; lipid fraction was determined by Soxhlet AOAC 920.39 method; crude protein by Kjeldahl AOAC 984.13 method; total dietary fiber (TDF) by enzymatic determi nation using a K-TDFR 05/12 Megazyme© enzymatic kit (Ireland) ac cording to AOAC 985.29 method; and lignin Klason content following TAPPI T222om-11 method.
2.5. Thermal analysis 2.5.1. Thermogravimetric analysis (TGA) Thermal degradation analysis was performed in a thermogravimetric balance TA Instrument Discovery Series (USA), through heating samples between 30 and 700 � C in air (25 mL min 1) at 10 � C.min 1. Weight versus temperature curves were recorded and the maximum decompo sition temperature of each film component was obtained from first de rivative curves. 2.5.2. Modulated differential scanning calorimetry (MDSC) Modulated differential scanning calorimetry (MDSC) assays were performed in a differential scanning calorimeter Q100 (TA Instruments, New Castle, USA). Approximately 10 mg of film were weighted in her metic pans in order to avoid water loss. An empty hermetic pan was used as reference. Samples were heated from 20 to 250 � C at 10 � C.min 1, under nitrogen atmosphere. From these thermograms, the following parameters were obtained: onset (T0) and melting (Tm) temperatures and enthalpy associated to this thermal transition (ΔHm).
2.1. Structural characterization of cassava and ahipa by-products Microstructure and morphology of by-products were studied by Scanning Electron Microscopy (SEM), using a JEOL JSM 6360 micro scope (Japan). Samples, mounted on bronze stubs using a double-sided tape and metalized with gold layer, were analyzed under high vacuum mode. Fourier transform infrared spectra were obtained using a Thermo Nicolet Nexus spectrophotometer (Milwaukee, USA). Samples were prepared by mixing residues as fine powder with KBr (Sigma–Aldrich, 99%) at 3 wt%. The mixtures were pressed and transparent samples were obtained. Spectra were achieved from 100 accumulated scans at 4 cm 1 resolution in the range 4000–400 cm 1.
2.5.3. Dynamic mechanical analysis Temperatures and intensities of relaxation phenomena for condi tioned TPS films were determined by DMTA, in a dynamic-mechanical thermal equipment Q800 (TA Instruments, New Castle, USA) with a liquid nitrogen cooling system, using tension clamps. Multi-frequency sweeps at fixed amplitude from 100 to 100 � C at 5 � C.min 1 were carried out. Storage (E0 ) and loss (E00 ) moduli and tan δ curves as a function of temperature were recorded and analyzed using the software Universal Analysis 2000. Temperatures of the relaxation processes associated to glass transition temperatures (Tg) were determined through the inflexion point of E0 curve, as well as, the maximum peak in both E00 and tan δ curves.
2.2. Thermoplastic starch mixtures Mixtures of corn starch, glycerol (30 wt %), distilled water (45 wt %), and residues (0.5 and 1.5 wt %) were prepared. Components concen trations were expressed in g per 100 g of starch. Formulations were named TPS, TPS-CP#, TPS-CB#, TPS-AP#, TPS-AB#, where # corre sponds to the percentage of the residue in the formulation (0.5 and 1.5 wt%). Residues were premixed with starch to achieve their good dispersion within the matrix. Then, glycerol and distilled water were added and the mixed samples were conditioned at 25 � C during 24 h. Mixtures were processed in a Brabender Plastograph (Brabender, Ger many) at 140 � C and 50 rpm for 15 min. Torque was monitored and recorded through the whole mixing process. From torque-time curves,
2.6. Optical properties 2.6.1. Opacity and UV barrier capacity Absorbance spectra (200–700 nm) were recorded using a U-1900 spectrophotometer (HITACHI, Japan). Films were cut into rectangles (3 � 1 cm) and placed on the internal side of a quartz spectrophotometer cell. Film transparency was estimated as the ratio between absorbance at 600 nm and film thickness and was expressed in mm 1, according to Piermaría et al. [30]. Blocking effect of studied fillers at 300, 350, and 750 nm was calculated as described in a previous work [31]. 2
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2.6.2. Color measurements Films color measurements were performed using a Konica colorim eter (CR 400, Osaka, Japan)Color parameters L, a, and b were recorded according to the CIELab scale, in at least ten randomly selected positions for each sample. Color parameters range from L ¼ 0 (black) to L ¼ 100 (white), -a (greenness) to þa (redness), and –b (blueness) to þb (yel lowness). Standard values considered were those of the white back ground (L ¼ 97.75, a ¼ 0.49, and b ¼ 1.96). Besides, ΔL, Δa and Δb were calculated, considering the standard values of the white back ground and the parameter color difference (ΔE) was also determined as follows: pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ΔE ¼ ΔL2 þ Δa2 þ Δb2 [1] 2.7. Water vapor permeability Water vapor permeability (WVP) was determined according to ASTM F 1249-89 standard method using a PERMATRAN-W® Model 3/33 (Mocon Inc., USA). Film sample was placed in a test cell which is divided into two chambers separated by the specimen. The inner chamber is filled with nitrogen (carrier gas) and the outer chamber with water vapor (test gas). Water molecules diffuse through the film to the inside chamber and water vapor transmission rate (WVTR) is registered. Measurements were carried out at 25 � C and films were subjected to a partial water vapor pressure gradient. Masked specimens with precut aluminum foil were used, leaving an uncovered film area of 5 cm2. From WVTR values obtained, WVP was calculated using the following equation: lxWVTR WVP ¼ Δp
Fig. 1. Scheme of starch extraction procedure. Yields are expressed as final dry weight of product and by-products.
[2]
Table 1 Chemical composition of cassava and ahipa by-products.
where l corresponds to film thickness (m) and Δp is the partial pressure difference across films (Pa). 2.8. Mechanical properties Tensile tests were performed in an Instron 3369 universal mechan ical testing system (Instron, USA) using a crosshead speed of 2 mm min 1 and a load cell of 50 N. Ten test specimens (13 � 100 mm) were assayed for each film formulation and stress-strain curves were calcu lated from load-displacement data. Young’s modulus (E), maximum P tensile strength (σm), and elongation at break ( b) were calculated according to ASTM D882-00 standard method.
Byproduct
TDFa (%)
Lipids (%)
Moisture (%)
Protein (%)
Ash (%)
Lignin (%)
Cassava peel (CP)b Cassava bagasse (CB)b Ahipa peel (AP) Ahipa bagasse (AB)c
55.14 � 0.82c
0.90 � 0.19b
9.67 � 0.03ab
7.05 � 1.09c
23.59 � 1.45c
24.18 � 2.78a
0.17 � 0.01a
10.56 � 1.58b
1.25 � 0.11a
10.70 � 0.12d 0.86 � 0.04a
52.47 � 0.15c 44.57 � 1.75b
1.81 � 0.13c 0.30 � 0.04a
7.89 � 0.02a 11.45 � 0.32b
13.41 � 0.11d 4.83 � 0.09b
7.44 � 0.16c 2.15 � 0.01b
25.67 � 1.96c 7.18 � 0.08b
1.62 � 0.58a
Reported values correspond to the mean � standard deviation. Different letters within the same columns indicate significant differences (p < 0.05) among samples. a TDF ¼ Total Dietary Fiber. b Data previously published in Versino et al. [9]. c Data previously published in L� opez et al. [57].
2.9. Statistical analysis A completely randomized experimental design was used to charac terize composite films, making an analysis of variance (ANOVA) to compare mean differences of samples properties: comparing mean values by the Fisher’s least significant difference test with a significance level p ¼ 0.05.
ash contents than peels, indicating a higher carbohydrates content attributed partially to total fiber and residual starch. Correspondingly, CB presented the lowest fiber content, indicating a higher starch residual content which had been reported in a previous work [9]. Furthermore, ahipa by-products presented in comparison higher protein and lipids contents than cassava ones. These results are in agreement with various studies on cassava and ahipa roots [18,20,27,32–34].
3. Results and discussion Starch extraction from tubers is a relatively simple procedure with three by-products: peels, bagasse, and wastewaters. In Fig. 1 a blocks flow diagram of the starch extraction form cassava and ahipa roots �pez et al. [18] is presented. Cassava roots process optimized by Lo showed a higher starch and solid by-products yield than ahipa, which is attributed to the fresh root moisture content difference (cassava: 67.35 � 4.8% and ahipa: 76.04 � 0.3%). Chemical composition of the stabilized residues is shown in Table 1. As expected, peels present a higher fiber and lignin content than bagasse, which in addition presented higher moisture contents. More over, both ahipa and cassava bagasse showed lower protein, lipids, and
3.1. Melt-mixing processing The simplest process for polymer composites production from ther moplastic polymers is melt-mixing the polymer, plasticizers, and fillers in suitable devices. The materials morphology obtained by melt-mixing is dependent on the mixing time and temperature, the shear gradient, 3
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plasticization energy, composite materials presented higher values than TPS.
and the rheological properties of the polymer [35]. The mixing torque is associated to the material viscosity during its processing and hence to the melt resistance to flow [36]. Since native starch melts above its degradation temperature plasticizers addition is essential for thermo plastic starch processing [37]. Particularly, in this work water and glycerol were used as plasticizers in order to reduce polymer chains intermolecular interactions and consequently reduce the material glass phase transition temperature (Tg) [38]. Maximum torque and plastici zation energy corresponding to the melt-mixing of composites based on thermoplastic corn starch and the organic fillers are shown in Fig. 2. When the mixture of starch, glycerol, and water was fed into the mixing chamber, granules caused a certain resistance to the blades’ rotation, increasing the torque up to 57 Nm and then it remained constant, obtaining a new homogeneous molten phase. Starch granules are dis rupted when heat transfer is sufficient for melting their core, facilitated by the presence of plasticizers [39]. Initial torque increase during pro cessing provides high shear force and facilitates the destruction of starch crystalline structure [40]. When this thermal process is completed, there are three possible scenarios: torque could decrease indicating chains degradation; it could increase indicating chains crosslinking; or it could stabilize, as it occurred in this case, indicating that neither crosslinking �rdoba et al. [28], nor degradation occurred. In accordance with Co plasticization energy can be calculated by integration of the torque–time curve within the time when torque starts to increase until the time where the maximum torque is reached. The authors stressed that it is in this region during mixing-process that the plasticizers diffuse into the starch granules. Thus, granules swell and absorb the plasticizer reaching a pseudo-solid state (maximum torque) before melting. Plasticization energy can be taken as an estimate of the thermodynamic favorability of the plasticizer diffusion into the polymer granules, and therefore a measure of the compatibility between them. Native corn TPS required a plasticization energy of 310 Nm.min (Fig. 2), however these are not comparable to values reported by other authors for different TPS com posite materials since diverse processing techniques and conditions were used [7,41,42]. The presence of bagasse and peels of both cassava and ahipa increased significantly (p < 0.05) the maximum torque reached during starch thermo-plasticization. This increment was more important for ahipa and cassava bagasse mainly due to the high residual starch content of these by-products. Similar results were reported by Vallejos et al. [41] for composites based on thermoplastic starch and fibers from the ethanol–water fractionation of bagasse. These authors suggested that the fiber–matrix interactions reduced the polymer chains mobility and increased the viscosity of the fiber–starch TPS blends. Regarding
3.2. Microstructural characterization SEM images of the fracture surfaces of TPS films with 1.5 wt% cas sava and ahipa by-products are shown in Fig. 3. Starch granules were not observed in any of the studied materials, demonstrating that starch structure disruption was not affected by bagasse nor peel addition dur ing TPS thermal processing. A similar observation was reported by Vallejos et al. [41] for TPS composites containing sugarcane bagasse fibers. In the case of TPS-CP and TPS-AP, peel is perceived as spots in SEM images and it was uniformly dispersed within TPS matrix (Fig. 2a and b). In addition, it could be seen that polymer matrix had fully covered the filler. Rani et al. [43] have informed similar results for TPS reinforced with cellulose nanofibers derived from sugarcane bagasse. Besides, no filler agglomerates were detected, which is indicative of an adequate dispersion efficiency. Moreover, no peel pull-out phenomenon was detected on fracture cross-sections of the composites, indicating a good adhesion and affinity of this filler with the TPS matrix [44,45]. Films containing bagasse presented rougher fracture surfaces than TPS-peel composites (Fig. 2c and d). However, no traces of bagasse
Fig. 3. SEM micrographs of composites based on thermoplastic starch (TPS) and 1.5 wt % of: a) cassava peel, b) ahipa peel, c) cassava bagasse, and d) ahipa bagasse.
Fig. 2. Maximum torque and plasticizing energy necessary in a Brabender Plastograph (Brabender, Germany) at 140 � C and 50 rpm for 15 min of composites based on thermoplastic cassava starch: control (TPS), and composites with 0.5 and 1.5 wt % of: cassava peel (CP), cassava bagasse (CB), ahipa peel (AP) and ahipa bagasse (AB). 4
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particles were visualized in any of TPS-bagasse composites, demon strating that these fillers were well-integrated within the matrix. Despite the wrinkled appearance, these composites presented a compact struc ture, in line with the study of thermoplastic cassava starch composite containing cassava bagasse carried out by Edhirej et al. [5].
Obtained results are presented in Table 2. Comparatively, films con taining fillers presented wider melting peaks (major differences between To and Tm) than TPS, probably because of the greater number of com ponents in composites formulations. In general, TPS composites with ahipa residues (AP and AB) presented higher melting peak temperatures (Tm) and more cooperative peaks than TPS composites reinforced with cassava residues (CP and CB), which could be attributed to a higher interaction among ahipa residue compounds and corn starch molecules.
3.3. Thermal analysis Thermal degradation of TPS based materials occurred in three stages as is shown in TGA curves (Fig. 4). The first step corresponds to the materials dehydration which occurs between 39.9 � 1.4 � C and 158.5 � 3.7 � C. All composites tested presented lower weight loss and higher decomposition temperatures than the control TPS films, with greater differences for higher filler contents. The second event corresponds mainly to starch molecules decomposition. For TPS films, it was observed a peak at 316 � C with a shoulder at 266 � C attributed to amylose and amylopectin differential degradation rate [46]. Filler addition did not modify the degradation step at 316 � C; however, an increase of nearly 20 � C was observed in the shoulder which could be a consequence of starch and filler interactions in composites matrix. Correspondingly, there was an increase (though slight) of the degrada tion temperature for 50% weight loss of the material for films containing organic fillers, indicating to some extent a higher thermal stability of composites. The greatest differences were observed in the third and last degradation step correlated to “glowing combustion” that occurs due to reactions of carbonaceous residues in the presence of oxygen, as it was �pez et al. [48]. This change was higher reported by Liu et al. [47] and Lo for all composites due to the higher solids content, and especially for samples reinforced with both cassava and ahipa bagasse, owing to their higher residual carbohydrates content (CP ¼ 71.68%, CB ¼ 87.16%, AP ¼ 69.45% and AB ¼ 81.27%). Two analysis were carried out to identify different phase transition phenomena: melting through DSC and glass transition by DMA.
Table 2 Thermal properties of composites based on thermoplastic starch (TPS) and cassava and ahipa by-products: peels (P) and bagasse (B). Composite TPS TPS-CP0.5 TPS-CP1.5 TPS-CB0.5 TPS-CB1.5 TPS-AP0.5 TPS-AP1.5 TPS-AB0.5 TPS-AB1.5
DSC
DMA
To (� C)
Tm (� C)
ΔHm (J/g)
Tg1 (� C)
Tg2 (� C)
133.1 � 1.3abc 103.9 � 19.6 ab 96.7 � 11.8a 109.9 � 24.5ab 102.8 � 10.5ab 160.1 � 21.8c 161.0 � 14.8c 139.7 � 19.7bc 167.7 � 4.8c
156.4 � 2.1a 164.0 � 16.8a 223.0 � 0.8c 156.5 � 18.1a 193.5 � 18.7b 229.0 � 16.5c 230.3 � 2.8c 226.2 � 8.4c 225.9 � 3.2c
140.7 � 32.2a 204.5 � 12.3cd 170.5 � 5.35abc 227.7 � 5.1d 205.7 � 10.9cd 158.9 � 10.7ab 192.5 � 19.5bcd 155.1 � 9.4ab 147.8 � 19.4a
43.90 � 1.7a ND
26.57 � 6.3a ND
35.75 � 4.5c ND
81.75 � 4.9e ND
39.00 � 2.9bc ND
53.55 � 5.6c ND
37.43 � 3.6c ND
71.45 � 1.6d ND
43.29 � 1.9ab
43.04 � 4.3b
Reported values correspond to the mean � standard deviation. Different letters within the same columns indicate significant differences (p < 0.05) among samples.
Fig. 4. Weight loss versus temperature curves by TGA of composites based on thermoplastic starch (TPS), with 0, 0.5 and 1.5 wt % of: a) cassava peel, b) ahipa peel, c) cassava bagasse, and d) ahipa bagasse. 5
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However, this effect was independent from the filler content (p > 0.05). Filler concentration on the other hand, had an impact on melting tem perature (Tm) and energy (ΔHm) of TPS with cassava by-products: higher filler content resulted in higher Tm though lower ΔHm. In general, however, an increase in melt enthalpy values of TPS composites was observed which could be indicative of an induction of crystallinity in the polymeric matrix due to the inclusion of fillers. The nucleating effect of filler agents have been proved on polymeric matrices such as PLA [49], although several studies on starch-based composites have shown different tendency depending on processing methods, filler type and content [8,44,50–52]. On TPS based films with filler contents lower than 10 wt % no significant effect on starch crystallization was reported [8, 44,51,52], yet further studies would be needed to confirm the main factors for the melt enthalpy variations herein reported. DMA results evidenced that, both glass transition temperatures (Tg1 and Tg2), espe cially the second transition corresponding to the starch-rich phase, were affected by filler type: TPS-CP > TPS-AP > TPS-CB > TPS-AB > TPS. This trend correlates with the filler’s fiber and lignin content (Table 1). 3.4. Optical, gas barrier and mechanical properties Color study revealed that films reinforced with cassava and ahipa peel presented the greatest differences with the control; being this effect proportional to the filler content (Fig. 5). These samples presented or ange shade in contrast to the more yellowish tone of TPS control (Fig. 5b). Color saturation and color differences were also affected by the filler type, though filler content had an insignificant or less important effect in TPS composites reinforced with cassava and ahipa bagasse (Fig. 5a and c). These results were attributed to the natural and char acteristic pigmentation of the fillers, especially of the peels. Moreover, the anthocyanins content of ahipa roots, relatively [53]vely high compared to cassava roots or other tubers [53], could have been partially retained in the ahipa bagasse particles and thus contributing to the higher color differences (Fig. 5). Since filler presence within TPS matrix can physically obstruct light, it would be expected that the incorporation of cassava and ahipa byproducts decreased films transparency and increased light blocking ef fect in all the spectrum (400–700 nm). However, the obtained results revealed that the transparency of the films was only affected with 1.5 wt % peel (Fig. 6). In correlation with color differences results, trans parency was more affected by cassava peel than ahipa one. It should be remarked though that, since the transparency parameter defined by Piermaria et al. [30] is proportional to the film absorbance at 600 nm, the higher this parameter the lower results the films transparency. As regards the charge blocking capacity it could be seen that within the visible range (750 nm) the tendency is the same as for the trans parency parameter (Fig. 6). Nonetheless, in the UV light range (350 and 300 nm) resulted to be quite independent of filler content (Fig. 6); this is reflected in the almost proportional decrease of the blocking parameter with filler content increase, meaning that the composites with 0.5 and 1.5 wt% of filler showed similar transmittance differences with de control. Among filler types, ahipa peel (AP) presented the higher UVlight blocking capacity, followed by cassava peel (CP), ahipa bagasse (AB) and cassava bagasse (CB), which correlates with the lignin content of the residues (Table 1). As shown in Table 3, all TPS composite films presented WVP values similar to those reported for other starch-based materials [54–56]. Even though the inclusion of fillers would be expected to increase tortuosity within the film matrix and therefore reduce water vapor diffusion, no significant differences (p > 0.05) were observed in WVP values for any composite regardless the filler type or content. These results are consistent with those reported previously for cassava starch films rein forced with cassava bagasse [9]. Mechanical behavior of TPS composites was significantly affected by filler type (Table 3). Cassava and ahipa bagasse incorporation gave place to more rigid and brittle materials, reflected in higher elastic moduli and
Fig. 5. Color parameters of composites based on thermoplastic starch (TPS), with 0, 0.5 and 1.5 wt % of cassava peel (CP), cassava bagasse (CB), ahipa peel (AP), and ahipa bagasse (AB): a) color saturation (Chroma*); b) color hue (Hue angle); and c) color difference (with a white standard). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
stress at break, and lower elongation capacity. On the contrary, cassava and ahipa peel reinforced TPS matrix, resulting in tougher materials with similar flexibility but higher strength (σm and E). Such effects were in all cases stressed by the filler content. 4. Conclusions The effect of low concentrations of organic fillers on thermoplastic corn starch properties was evaluated. As fillers it was employed bagasse and peel obtained during starch extraction from cassava and ahipa tu bers. Overall, results demonstrated a strong effect of filler type on TPS composite properties, even at low concentrations. In general, peel had a greater impact on thermal, mechanical, and optical properties of TPS films. However, thermal stability nor water vapor permeability of the samples were affected by the presence of studied fillers. In conclusion, it was demonstrated the feasibility of using tubers’ peel and bagasse as fillers of biodegradable polymer films in order to modify their properties 6
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Fig. 6. Light barrier capacity of composites based on thermoplastic starch (TPS), with 0, 0.5 and 1.5 wt % of cassava peel (CP), cassava bagasse (CB), ahipa peel (AP), and ahipa bagasse (AB): blocking effect at 750, 350 and 300 nm (Δ%T/%weight filler charge), and transparency (mm 1 � 102).
References
Table 3 Tensile mechanical resistance and water vapor permeability of composites based on thermoplastic starch (TPS) and cassava and ahipa by-products: peels (P) and bagasse (B). Composite
Mechanical properties P σm (MPa) b (%)
TPS
62.6 � 4.1bc 76.5 � 7.1d 64.3 � 9.9bc 61.7 � 4.5b 41.1 � 5.1a 73.2 � 8.6cd 73.5 � 6.5cd 55.7 � 8.8b 33.7 � 7.3a
TPS-CP0.5 TPS-CP1.5 TPS-CB0.5 TPS-CB1.5 TPS-AP0.5 TPS-AP1.5 TPS-AB0.5 TPS-AB1.5
E (Mpa)
WVP x 10 Pa)
10
1.19 � 0.04a 2.6 � 0.08b
22.7 � 1.7a
1.36 � 0.05a
78.2 � 9.7b
1.34 � 0.25a
3.0 � 0.15c
96.8 � 14.2b
1.30 � 0.01a
5.0 � 0.51d
192.3 � 31.2cd 217.9 � 13.4d 81.2 � 10.7b
1.45 � 0.20a
97.5 � 14.8b
1.41 � 0.10a
5.0 � 0.26d
182.0 � 30.2c
1.24 � 0.03a
5.1 � 0.51de
203.3 � 31.7
1.35 � 0.03a
5.5 � 0.41e 2.8 � 0.20bc 3.1 � 0.28c
cd
[1] Flauzino Neto WP, Silv� erio HA, Dantas NO, Pasquini D. Extraction and characterization of cellulose nanocrystals from agro-industrial residue - soy hulls. Ind Crops Prod 2013;42:480–8. https://doi.org/10.1016/j.indcrop.2012.06.041. [2] Wool R, Sun XS. Bio-based polymers and composites. Elsevier Academic Press; 2005. https://doi.org/10.1016/B978-0-12-763952-9.X5000-X. [3] Bodirlau R, Teaca C-A, Resmerita A-M, Spiridon I. Investigation of structural and thermal properties of different wood species treated with toluene-2,4-diisocyanate. Cellul Chem Technol 2012;46:381–7. [4] Gilfillan WN, Moghaddam L, Doherty WOS. Preparation and characterization of composites from starch with sugarcane bagasse nanofibres. Cellulose 2014;21: 2695–712. https://doi.org/10.1007/s10570-014-0277-4. [5] Edhirej A, Sapuan SM, Jawaid M, Zahari NI. Preparation and characterization of cassava bagasse reinforced thermoplastic cassava starch. Fibers Polym 2017;18: 162–71. https://doi.org/10.1007/s12221-017-6251-7. [6] Kuciel S, Ku�zniar P, Liber-Kne�c a. Polymer biocomposites with renewable sources. Arch Foundry Eng 2010;10:53–6. [7] Casta~ no J, Bouza R, Rodríguez-Llamazares S, Carrasco C, Vinicius RVB. Processing and characterization of starch-based materials from pehuen seeds (Araucaria araucana (Mol) K. Koch). Carbohydr Polym 2012;88:299–307. https://doi.org/ 10.1016/j.carbpol.2011.12.008. [8] Teixeira E de M, Pasquini D, Curvelo AAS, Corradini E, Belgacem MN, Dufresne A. Cassava bagasse cellulose nanofibrils reinforced thermoplastic cassava starch. Carbohydr Polym 2009;78:422–31. https://doi.org/10.1016/j. carbpol.2009.04.034. [9] Versino F, L� opez OV, García MA. Sustainable use of cassava (Manihot esculenta) roots as raw material for biocomposites development. Ind Crops Prod 2015;65: 79–89. https://doi.org/10.1016/j.indcrop.2014.11.054. [10] Pasquini D, Teixeira E de M, Curvelo AA da S, Belgacem MN, Dufresne A. Extraction of cellulose whiskers from cassava bagasse and their applications as reinforcing agent in natural rubber. Ind Crops Prod 2010;32:486–90. https://doi. org/10.1016/j.indcrop.2010.06.022. [11] Scott G, Best R, Rosegrant M, Bokanga M. Roots and tubers in the global food system_A vision statement to the year 2020. Lima, Perú: International Potato Center; 2000. [12] Gao Y, Duan A, Qiu X, Liu Z, Sun J, Zhang J, et al. Distribution of roots and root length density in a maize/soybean strip intercropping system. Agric Water Manag 2010. https://doi.org/10.1016/j.agwat.2010.08.021. [13] Iftekhar H, Riaz A, Abdul J, Nazir M, Mah-mood T. Competitive behaviour of component crops in different sesame-legume intercropping systems. J Agric Biolology 2006;8:165–7. [14] Howeler RH. Sustainable cassava production in Asia for multiple uses and for multiple markets. Proc. ninth Reg. Work. held Nanning, Guangxi, China PR 2011: 336. [15] Giller KE. Nitrogen fixation in tropical cropping systems. second ed. New York, USA: CABI Publishing; 2001. https://doi.org/10.1016/0378-4290(93)90012-c. [16] Crews TE, Peoples MB. Legume versus fertilizer sources of nitrogen: ecological tradeoffs and human needs. Agric Ecosyst Environ 2004;102:279–97. https://doi. org/10.1016/j.agee.2003.09.018. [17] Rusinamhodzi L, Corbeels M, Nyamangara J, Giller KE. Maize-grain legume intercropping is an attractive option for ecological intensification that reduces climatic risk for smallholder farmers in central Mozambique. Field Crop Res 2012; 136:12–22. https://doi.org/10.1016/j.fcr.2012.07.014.
(g/m s
1.56 � 0.31a 1.34 � 0.08a
Reported values correspond to the mean � standard deviation. Different letters within the same columns indicate significant differences (p < 0.05) among samples.
and to give added value to these agroindustry by-products. Acknowledgements �n This work was supported by the Agencia Nacional de Promocio �gica (ANPCyT, Project PICT 2011–1213, Científica y Tecnolo 2015–0921, 2014–2410) and the Consejo Nacional de Investigaciones Científicas y T� ecnicas (CONICET). Florencia Versino wishes to thank CONICET as well for a Doctoral and Postdoctoral Fellowship. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.compositesb.2019.107653.
7
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[18] L� opez OV, Vi~ na SZ, Pachas ANA, Sisterna MN, Rohatsch PH, Mugridge A, et al. Composition and food properties of Pachyrhizus ahipa roots and starch. Int J Food Sci Technol 2010;45:223–33. https://doi.org/10.1111/j.1365-2621.2009.02125.x. [19] L� opez OV, García MA. Starch films from a novel (Pachyrhizus ahipa) and conventional sources: development and characterization. Mater Sci Eng C 2012;32: 1931–40. https://doi.org/10.1016/j.msec.2012.05.035. [20] Dini C, Doporto MC, García MA, Vi~ na SZ. Nutritional profile and anti-nutrient analyses of Pachyrhizus ahipa roots from different accessions. Food Res Int 2013; 54:255–61. https://doi.org/10.1016/j.foodres.2013.07.027. [21] Ghimire A, Sen R, Annachhatre AP. Biosolid management options in cassava starch industries of Thailand: present practice and future possibilities. Procedia Chem 2015;14:66–75. https://doi.org/10.1016/j.proche.2015.03.011. [22] Suoto LRF, Caliari M, Soares Júnior MS, Fiorda FA, Garcia MC. Utilization of residue from cassava starch processing for production of fermentable sugar by enzymatic hydrolysis. Food Sci Technol 2016;37:19–24. https://doi.org/10.1590/ 1678-457x.0023. [23] Pandey A, Soccol CR, Nigam P, Soccol VT, Vandenberghe LP, Mohan R. Biotechnological potential of agro-industrial residues. II: cassava bagasse. Bioresour Technol 2000;74:81–7. https://doi.org/10.1016/S0960-8524(99) 00143-1. [24] Jyothi AN, Sasikiran K, Nambisan B, Balagopalan C. Optimisation of glutamic acid production from cassava starch factory residues using Brevibacterium divaricatum. Process Biochem 2005;40:3576–9. https://doi.org/10.1016/j. procbio.2005.03.046. [25] V€ ais€ anen T, Haapala A, Lappalainen R, Tomppo L. Utilization of agricultural and forest industry waste and residues in natural fiber-polymer composites: a review. Waste Manag 2016. https://doi.org/10.1016/j.wasman.2016.04.037. [26] L� opez OV, García MA, Zaritzky NE. Film forming capacity of chemically modified corn starches. Carbohydr Polym 2008;73:573–81. https://doi.org/10.1016/j. carbpol.2007.12.023. [27] Lopez OV, Versino F, Villar MA, Garcia MA. Agro-industrial residue from starch extraction of Pachyrhizus ahipa as filler of thermoplastic corn starch films. Carbohydr Polym 2015;134:324–32. https://doi.org/10.1016/j. carbpol.2015.07.081. [28] C� ordoba A, Cu�ellar N, Gonz� alez M, Medina J. The plasticizing effect of alginate on the thermoplastic starch/glycerin blends. Carbohydr Polym 2008;73:409–16. https://doi.org/10.1016/j.carbpol.2007.12.007. [29] Castillo L, L� opez OV, L� opez C, Zaritzky N, García MA, Barbosa S, et al. Thermoplastic starch films reinforced with talc nanoparticles. Carbohydr Polym 2013;95:664–74. https://doi.org/10.1016/j.carbpol.2013.03.026. [30] Piermaria J, Bosch A, Pinotti A, Yantorno O, Garcia MA, Abraham AG. Kefiran films plasticized with sugars and polyols: water vapor barrier and mechanical properties in relation to their microstructure analyzed by ATR/FT-IR spectroscopy. Food Hydrocolloids 2011;25:1261–9. https://doi.org/10.1016/j. foodhyd.2010.11.024. [31] Mbey JA, Hoppe S, Thomas F. Cassava starch–kaolinite composite film. Effect of clay content and clay modification on film properties. Carbohydr Polym 2012;88: 213–22. https://doi.org/10.1016/j.carbpol.2011.11.091. [32] Abera T, Admasu S, Desse G. Effect of processing on physicochemical compositon of Cassava_Cyanide and other antinutrients in cassava. Saarbücken. Germany: VDM Verlag Dr. Müller GmdbH & Co. KG; 2010. [33] Doporto MC. Aprovechamiento integral de raíces de ahipa ( Pachyrhizus ahipa) y sus productos derivados con fines alimentarios. 2014. [34] Dini C, García MA, Vi~ na SZ. Non-traditional flours: frontiers between ancestral heritage and innovation. Food Funct 2012;3:606–20. https://doi.org/10.1039/ c2fo30036b. [35] Braun DB, Rose MR. Rheology modifiers handbook: practical use and applilcation. William Andrew; 2013. [36] da Silva MC, Ramírez Ascheri DP, Piler de Carvalho CW, Galdeano MC, Trist~ ao de Andrade C. Characterization of cassava starch processed in an internal mixer. Polímeros - Ci^ encia Tecnol 2013;23:825–32. https://doi.org/10.4322/ polimeros.2014.007. [37] Galdeano MC, Mali S, Grossmann MVE, Yamashita F, García MA. Effects of plasticizers on the properties of oat starch films. Mater Sci Eng C 2009;29:532–8. https://doi.org/10.1016/j.msec.2008.09.034.
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Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Exploitation of by-products from cassava and ahipa starch extraction as filler of thermoplastic corn starch �pez b, M. Alejandra García a, d Versino Florencia a, c, *, Olivia V. Lo a
Centro de Investigaci� on y Desarrollo en Criotecnología de Alimentos, CIDCA (UNLP-CONICET-CICPBA), Facultad de Ciencias Exactas, UNLP, 47 y 116, (1900), La Plata, Argentina b Planta Piloto de Ingeniería Química, PLAPIQUI (UNS-CONICET), Departamento de Ingeniería Química, UNS, Camino La Carrindanga km. 7, (8000), Bahía Blanca, Argentina c Departamento de Ingeniería Química, Facultad de Ingeniería, Universidad Nacional de La Plata (UNLP), 48 y 115, La Plata, Buenos Aires, 1900, Argentina d Departamento de Química, Facultad de Ciencias Exactas, Universidad Nacional de La Plata (UNLP), 47 y 115, La Plata, Buenos Aires, 1900, Argentina
A R T I C L E I N F O
A B S T R A C T
Keywords: Biocomposites Sustainable materials Biodegradable Agroindustry by-products
Advances in bio-based composite materials offer potential opportunities for economic revalorization of agro industrial residues such as starchy tubers peels and bagasse. In this work, cassava and ahipa peels and bagasse are studied as potential fillers of thermoplastic corn starch (TPS) films. It was assessed the effect of studied organic fillers on thermal stability as well as optical, water vapor barrier, and mechanical properties of TPS. In all cases, type (peel or bagasse) and content (0.5 and 1.5 wt %) of the filler had a great impact on the biocomposites properties. Results demonstrated that these fillers modify TPS properties and gives utility to these residues with low economic cost.
1. Introduction Environmental problems associated with synthetic plastics and the exhaustion of fossil resources have led to a great interest in sustainable and ecologically friendly materials [1]. Biomass conversion into valu able materials like bio-based polymers and biocomposites has both substantial economic and environmental relevance [2]. Throughout the last decades numerous developments on biocomposites have been car ried out making them interesting alternatives for conventional materials in a vast number of end-uses. Recently, natural fibers are increasingly being used as reinforcing materials in polymers and composites. In comparison to inorganic fillers, lignocellulosic fillers are more attractive owing to their renewable nature, biodegradability, low energy con sumption, wide availability, low cost and density, and high specific strength and modulus [3,4]. Agroindustrial or agricultural residues can be considered important for natural fibers obtention [5]. Several authors have reported the use of natural fibers as fillers of starch biocomposites [6,7]. Particularly, the addition of peels and bagasse in starch compos ites development has also been reported by Teixeira et al. [8], Versino et al. [9], and Pasquini et al. [10]. Moreover, root and tuber crops are important to agriculture, food security, and income for 2.2 billion people in developing countries [11].
Among these crops, cassava (Manihot esculenta) is ranked as the fifth most widely used starch source in the world, and third among the food sources consumed in tropical regions [5]. Cassava is a robust crop that grows well in poor soils and in areas with low or unpredictable rainfall. However, to longer sustain high yields, it is important to prevent soil nutrient depletion and erosion, which can be achieved through simple agronomic or soil conservation practices such as intercropping [12–14]. In this regard, legumes such as Pachyrhizus ahipa, also called ahipa or Andean yam bean, are considered good alternatives to be intercropped with cassava since they improve soil fertility [15–20]. Both, cassava and ahipa tubers, accumulate starch in large quantities, hence representing a highly energetic food source. Starch production processes from roots and tubers is energy as well as water intensive and generates wastewater and solid waste (peels and bagasse) with high organic load which leads to significant water as well as air pollution [21,22]. Pandey et al. [23] reported that processing of 250–300 ton of cassava tubers results in about 1.6 ton of solid peels and about 280 ton of bagasse with a high moisture content (85%). Solid wastes are generally discarded in the environment in landfills without any treatment. Besides being used as animal feed or burned for energy purposes, no other practical applica tions have been found for these residues [24]. Therefore, there is an increasing interest in finding new options to produce value-added
* Corresponding author. E-mail address: [email protected] (V. Florencia). https://doi.org/10.1016/j.compositesb.2019.107653 Received 20 July 2019; Received in revised form 25 October 2019; Accepted 28 November 2019 Available online 29 November 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Versino Florencia, Composites Part B, https://doi.org/10.1016/j.compositesb.2019.107653
V. Florencia et al.
Composites Part B xxx (xxxx) xxx
products from these solid by-products. Accordingly, the combination of environmental awareness, economic drivers, and international policies encourage the recuperation and reutilization of agricultural and forest industry waste components [25]. A possible exploitation of peels and bagasse obtained from cassava and ahipa tubers is the use of these by-products as fillers of polymeric matrices, especially those based on bio-based and biodegradable polymers. Consequently, the aim of this work was to give added value to byproducts (peels and bagasse) derived from cassava and ahipa starch extraction by using them as fillers of thermoplastic corn starch films. Processing techniques employed to obtain these biomaterials were meltmixing and thermocompression. The effect of the presence of these organic fillers on the thermal plasticization of corn starch, as well as their influence on microstructure and final properties of the starch biodegradable films were evaluated.
maximum torque and plasticization energy were determined as it was �rdoba et al. [28]. proposed by Co
2. Materials and methods
Homogeneity and appearance of films were examined by Scanning Electron Microscopy (SEM), using a JEOL JSM-35 CF electron micro scope (Japan), with a secondary electron detector. Films were cryofractured by immersion in liquid nitrogen, mounted on bronze stubs and coated with a gold layer, using an argon plasma metallizer (sputter coater PELCO 91000).
2.3. Films preparation Thermoplastic starch films were obtained by thermo-compression using a hydraulic press, following the processing conditions reported in a previous work [29]. Mixtures were conditioned at 25 � C and 60% relative humidity (RH) previous to thermo-compression (140 � C, 150 kg cm 2, 6 min). Before characterization, films were conditioned again at 25 � C and 60% RH. Film thickness was measured at least in ten different locations with a digital coating thickness gauge CM-8822 (SolTec, Argentina). 2.4. Microstructural characterization
�n, Native corn starch was provided by Misky–Arcor (Tucuma Argentina) with an amylose content of 23.9 � 0.7% [26]. Cassava and ahipa peels and bagasse were obtained at laboratory scale using an �pez et al. [18] optimized method for starch extraction proposed by Lo from tubers of plants cultivated at INTA Montecarlo farm (Misiones, Argentina). These residues, named as cassava peel (CP), cassava bagasse (CB), ahipa peel (AP), and ahipa bagasse (AB), were dried at 50 � C to constant weight, ground, and sieved (through a 500 μm standardized mesh). Residues yield was calculated gravimetrically and expressed as g of dry by-product per 100 g of roots (wt.%) and reported values corre spond to the mean of three determinations for each residue. They were chemically characterized following standard methods as reported in previous works [9,27]. In brief, quantification of moisture and total ash content were determined gravimetrically at 105 � C until constant weight and according to AOAC 923.03 method, respectively; lipid fraction was determined by Soxhlet AOAC 920.39 method; crude protein by Kjeldahl AOAC 984.13 method; total dietary fiber (TDF) by enzymatic determi nation using a K-TDFR 05/12 Megazyme© enzymatic kit (Ireland) ac cording to AOAC 985.29 method; and lignin Klason content following TAPPI T222om-11 method.
2.5. Thermal analysis 2.5.1. Thermogravimetric analysis (TGA) Thermal degradation analysis was performed in a thermogravimetric balance TA Instrument Discovery Series (USA), through heating samples between 30 and 700 � C in air (25 mL min 1) at 10 � C.min 1. Weight versus temperature curves were recorded and the maximum decompo sition temperature of each film component was obtained from first de rivative curves. 2.5.2. Modulated differential scanning calorimetry (MDSC) Modulated differential scanning calorimetry (MDSC) assays were performed in a differential scanning calorimeter Q100 (TA Instruments, New Castle, USA). Approximately 10 mg of film were weighted in her metic pans in order to avoid water loss. An empty hermetic pan was used as reference. Samples were heated from 20 to 250 � C at 10 � C.min 1, under nitrogen atmosphere. From these thermograms, the following parameters were obtained: onset (T0) and melting (Tm) temperatures and enthalpy associated to this thermal transition (ΔHm).
2.1. Structural characterization of cassava and ahipa by-products Microstructure and morphology of by-products were studied by Scanning Electron Microscopy (SEM), using a JEOL JSM 6360 micro scope (Japan). Samples, mounted on bronze stubs using a double-sided tape and metalized with gold layer, were analyzed under high vacuum mode. Fourier transform infrared spectra were obtained using a Thermo Nicolet Nexus spectrophotometer (Milwaukee, USA). Samples were prepared by mixing residues as fine powder with KBr (Sigma–Aldrich, 99%) at 3 wt%. The mixtures were pressed and transparent samples were obtained. Spectra were achieved from 100 accumulated scans at 4 cm 1 resolution in the range 4000–400 cm 1.
2.5.3. Dynamic mechanical analysis Temperatures and intensities of relaxation phenomena for condi tioned TPS films were determined by DMTA, in a dynamic-mechanical thermal equipment Q800 (TA Instruments, New Castle, USA) with a liquid nitrogen cooling system, using tension clamps. Multi-frequency sweeps at fixed amplitude from 100 to 100 � C at 5 � C.min 1 were carried out. Storage (E0 ) and loss (E00 ) moduli and tan δ curves as a function of temperature were recorded and analyzed using the software Universal Analysis 2000. Temperatures of the relaxation processes associated to glass transition temperatures (Tg) were determined through the inflexion point of E0 curve, as well as, the maximum peak in both E00 and tan δ curves.
2.2. Thermoplastic starch mixtures Mixtures of corn starch, glycerol (30 wt %), distilled water (45 wt %), and residues (0.5 and 1.5 wt %) were prepared. Components concen trations were expressed in g per 100 g of starch. Formulations were named TPS, TPS-CP#, TPS-CB#, TPS-AP#, TPS-AB#, where # corre sponds to the percentage of the residue in the formulation (0.5 and 1.5 wt%). Residues were premixed with starch to achieve their good dispersion within the matrix. Then, glycerol and distilled water were added and the mixed samples were conditioned at 25 � C during 24 h. Mixtures were processed in a Brabender Plastograph (Brabender, Ger many) at 140 � C and 50 rpm for 15 min. Torque was monitored and recorded through the whole mixing process. From torque-time curves,
2.6. Optical properties 2.6.1. Opacity and UV barrier capacity Absorbance spectra (200–700 nm) were recorded using a U-1900 spectrophotometer (HITACHI, Japan). Films were cut into rectangles (3 � 1 cm) and placed on the internal side of a quartz spectrophotometer cell. Film transparency was estimated as the ratio between absorbance at 600 nm and film thickness and was expressed in mm 1, according to Piermaría et al. [30]. Blocking effect of studied fillers at 300, 350, and 750 nm was calculated as described in a previous work [31]. 2
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2.6.2. Color measurements Films color measurements were performed using a Konica colorim eter (CR 400, Osaka, Japan)Color parameters L, a, and b were recorded according to the CIELab scale, in at least ten randomly selected positions for each sample. Color parameters range from L ¼ 0 (black) to L ¼ 100 (white), -a (greenness) to þa (redness), and –b (blueness) to þb (yel lowness). Standard values considered were those of the white back ground (L ¼ 97.75, a ¼ 0.49, and b ¼ 1.96). Besides, ΔL, Δa and Δb were calculated, considering the standard values of the white back ground and the parameter color difference (ΔE) was also determined as follows: pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ΔE ¼ ΔL2 þ Δa2 þ Δb2 [1] 2.7. Water vapor permeability Water vapor permeability (WVP) was determined according to ASTM F 1249-89 standard method using a PERMATRAN-W® Model 3/33 (Mocon Inc., USA). Film sample was placed in a test cell which is divided into two chambers separated by the specimen. The inner chamber is filled with nitrogen (carrier gas) and the outer chamber with water vapor (test gas). Water molecules diffuse through the film to the inside chamber and water vapor transmission rate (WVTR) is registered. Measurements were carried out at 25 � C and films were subjected to a partial water vapor pressure gradient. Masked specimens with precut aluminum foil were used, leaving an uncovered film area of 5 cm2. From WVTR values obtained, WVP was calculated using the following equation: lxWVTR WVP ¼ Δp
Fig. 1. Scheme of starch extraction procedure. Yields are expressed as final dry weight of product and by-products.
[2]
Table 1 Chemical composition of cassava and ahipa by-products.
where l corresponds to film thickness (m) and Δp is the partial pressure difference across films (Pa). 2.8. Mechanical properties Tensile tests were performed in an Instron 3369 universal mechan ical testing system (Instron, USA) using a crosshead speed of 2 mm min 1 and a load cell of 50 N. Ten test specimens (13 � 100 mm) were assayed for each film formulation and stress-strain curves were calcu lated from load-displacement data. Young’s modulus (E), maximum P tensile strength (σm), and elongation at break ( b) were calculated according to ASTM D882-00 standard method.
Byproduct
TDFa (%)
Lipids (%)
Moisture (%)
Protein (%)
Ash (%)
Lignin (%)
Cassava peel (CP)b Cassava bagasse (CB)b Ahipa peel (AP) Ahipa bagasse (AB)c
55.14 � 0.82c
0.90 � 0.19b
9.67 � 0.03ab
7.05 � 1.09c
23.59 � 1.45c
24.18 � 2.78a
0.17 � 0.01a
10.56 � 1.58b
1.25 � 0.11a
10.70 � 0.12d 0.86 � 0.04a
52.47 � 0.15c 44.57 � 1.75b
1.81 � 0.13c 0.30 � 0.04a
7.89 � 0.02a 11.45 � 0.32b
13.41 � 0.11d 4.83 � 0.09b
7.44 � 0.16c 2.15 � 0.01b
25.67 � 1.96c 7.18 � 0.08b
1.62 � 0.58a
Reported values correspond to the mean � standard deviation. Different letters within the same columns indicate significant differences (p < 0.05) among samples. a TDF ¼ Total Dietary Fiber. b Data previously published in Versino et al. [9]. c Data previously published in L� opez et al. [57].
2.9. Statistical analysis A completely randomized experimental design was used to charac terize composite films, making an analysis of variance (ANOVA) to compare mean differences of samples properties: comparing mean values by the Fisher’s least significant difference test with a significance level p ¼ 0.05.
ash contents than peels, indicating a higher carbohydrates content attributed partially to total fiber and residual starch. Correspondingly, CB presented the lowest fiber content, indicating a higher starch residual content which had been reported in a previous work [9]. Furthermore, ahipa by-products presented in comparison higher protein and lipids contents than cassava ones. These results are in agreement with various studies on cassava and ahipa roots [18,20,27,32–34].
3. Results and discussion Starch extraction from tubers is a relatively simple procedure with three by-products: peels, bagasse, and wastewaters. In Fig. 1 a blocks flow diagram of the starch extraction form cassava and ahipa roots �pez et al. [18] is presented. Cassava roots process optimized by Lo showed a higher starch and solid by-products yield than ahipa, which is attributed to the fresh root moisture content difference (cassava: 67.35 � 4.8% and ahipa: 76.04 � 0.3%). Chemical composition of the stabilized residues is shown in Table 1. As expected, peels present a higher fiber and lignin content than bagasse, which in addition presented higher moisture contents. More over, both ahipa and cassava bagasse showed lower protein, lipids, and
3.1. Melt-mixing processing The simplest process for polymer composites production from ther moplastic polymers is melt-mixing the polymer, plasticizers, and fillers in suitable devices. The materials morphology obtained by melt-mixing is dependent on the mixing time and temperature, the shear gradient, 3
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plasticization energy, composite materials presented higher values than TPS.
and the rheological properties of the polymer [35]. The mixing torque is associated to the material viscosity during its processing and hence to the melt resistance to flow [36]. Since native starch melts above its degradation temperature plasticizers addition is essential for thermo plastic starch processing [37]. Particularly, in this work water and glycerol were used as plasticizers in order to reduce polymer chains intermolecular interactions and consequently reduce the material glass phase transition temperature (Tg) [38]. Maximum torque and plastici zation energy corresponding to the melt-mixing of composites based on thermoplastic corn starch and the organic fillers are shown in Fig. 2. When the mixture of starch, glycerol, and water was fed into the mixing chamber, granules caused a certain resistance to the blades’ rotation, increasing the torque up to 57 Nm and then it remained constant, obtaining a new homogeneous molten phase. Starch granules are dis rupted when heat transfer is sufficient for melting their core, facilitated by the presence of plasticizers [39]. Initial torque increase during pro cessing provides high shear force and facilitates the destruction of starch crystalline structure [40]. When this thermal process is completed, there are three possible scenarios: torque could decrease indicating chains degradation; it could increase indicating chains crosslinking; or it could stabilize, as it occurred in this case, indicating that neither crosslinking �rdoba et al. [28], nor degradation occurred. In accordance with Co plasticization energy can be calculated by integration of the torque–time curve within the time when torque starts to increase until the time where the maximum torque is reached. The authors stressed that it is in this region during mixing-process that the plasticizers diffuse into the starch granules. Thus, granules swell and absorb the plasticizer reaching a pseudo-solid state (maximum torque) before melting. Plasticization energy can be taken as an estimate of the thermodynamic favorability of the plasticizer diffusion into the polymer granules, and therefore a measure of the compatibility between them. Native corn TPS required a plasticization energy of 310 Nm.min (Fig. 2), however these are not comparable to values reported by other authors for different TPS com posite materials since diverse processing techniques and conditions were used [7,41,42]. The presence of bagasse and peels of both cassava and ahipa increased significantly (p < 0.05) the maximum torque reached during starch thermo-plasticization. This increment was more important for ahipa and cassava bagasse mainly due to the high residual starch content of these by-products. Similar results were reported by Vallejos et al. [41] for composites based on thermoplastic starch and fibers from the ethanol–water fractionation of bagasse. These authors suggested that the fiber–matrix interactions reduced the polymer chains mobility and increased the viscosity of the fiber–starch TPS blends. Regarding
3.2. Microstructural characterization SEM images of the fracture surfaces of TPS films with 1.5 wt% cas sava and ahipa by-products are shown in Fig. 3. Starch granules were not observed in any of the studied materials, demonstrating that starch structure disruption was not affected by bagasse nor peel addition dur ing TPS thermal processing. A similar observation was reported by Vallejos et al. [41] for TPS composites containing sugarcane bagasse fibers. In the case of TPS-CP and TPS-AP, peel is perceived as spots in SEM images and it was uniformly dispersed within TPS matrix (Fig. 2a and b). In addition, it could be seen that polymer matrix had fully covered the filler. Rani et al. [43] have informed similar results for TPS reinforced with cellulose nanofibers derived from sugarcane bagasse. Besides, no filler agglomerates were detected, which is indicative of an adequate dispersion efficiency. Moreover, no peel pull-out phenomenon was detected on fracture cross-sections of the composites, indicating a good adhesion and affinity of this filler with the TPS matrix [44,45]. Films containing bagasse presented rougher fracture surfaces than TPS-peel composites (Fig. 2c and d). However, no traces of bagasse
Fig. 3. SEM micrographs of composites based on thermoplastic starch (TPS) and 1.5 wt % of: a) cassava peel, b) ahipa peel, c) cassava bagasse, and d) ahipa bagasse.
Fig. 2. Maximum torque and plasticizing energy necessary in a Brabender Plastograph (Brabender, Germany) at 140 � C and 50 rpm for 15 min of composites based on thermoplastic cassava starch: control (TPS), and composites with 0.5 and 1.5 wt % of: cassava peel (CP), cassava bagasse (CB), ahipa peel (AP) and ahipa bagasse (AB). 4
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particles were visualized in any of TPS-bagasse composites, demon strating that these fillers were well-integrated within the matrix. Despite the wrinkled appearance, these composites presented a compact struc ture, in line with the study of thermoplastic cassava starch composite containing cassava bagasse carried out by Edhirej et al. [5].
Obtained results are presented in Table 2. Comparatively, films con taining fillers presented wider melting peaks (major differences between To and Tm) than TPS, probably because of the greater number of com ponents in composites formulations. In general, TPS composites with ahipa residues (AP and AB) presented higher melting peak temperatures (Tm) and more cooperative peaks than TPS composites reinforced with cassava residues (CP and CB), which could be attributed to a higher interaction among ahipa residue compounds and corn starch molecules.
3.3. Thermal analysis Thermal degradation of TPS based materials occurred in three stages as is shown in TGA curves (Fig. 4). The first step corresponds to the materials dehydration which occurs between 39.9 � 1.4 � C and 158.5 � 3.7 � C. All composites tested presented lower weight loss and higher decomposition temperatures than the control TPS films, with greater differences for higher filler contents. The second event corresponds mainly to starch molecules decomposition. For TPS films, it was observed a peak at 316 � C with a shoulder at 266 � C attributed to amylose and amylopectin differential degradation rate [46]. Filler addition did not modify the degradation step at 316 � C; however, an increase of nearly 20 � C was observed in the shoulder which could be a consequence of starch and filler interactions in composites matrix. Correspondingly, there was an increase (though slight) of the degrada tion temperature for 50% weight loss of the material for films containing organic fillers, indicating to some extent a higher thermal stability of composites. The greatest differences were observed in the third and last degradation step correlated to “glowing combustion” that occurs due to reactions of carbonaceous residues in the presence of oxygen, as it was �pez et al. [48]. This change was higher reported by Liu et al. [47] and Lo for all composites due to the higher solids content, and especially for samples reinforced with both cassava and ahipa bagasse, owing to their higher residual carbohydrates content (CP ¼ 71.68%, CB ¼ 87.16%, AP ¼ 69.45% and AB ¼ 81.27%). Two analysis were carried out to identify different phase transition phenomena: melting through DSC and glass transition by DMA.
Table 2 Thermal properties of composites based on thermoplastic starch (TPS) and cassava and ahipa by-products: peels (P) and bagasse (B). Composite TPS TPS-CP0.5 TPS-CP1.5 TPS-CB0.5 TPS-CB1.5 TPS-AP0.5 TPS-AP1.5 TPS-AB0.5 TPS-AB1.5
DSC
DMA
To (� C)
Tm (� C)
ΔHm (J/g)
Tg1 (� C)
Tg2 (� C)
133.1 � 1.3abc 103.9 � 19.6 ab 96.7 � 11.8a 109.9 � 24.5ab 102.8 � 10.5ab 160.1 � 21.8c 161.0 � 14.8c 139.7 � 19.7bc 167.7 � 4.8c
156.4 � 2.1a 164.0 � 16.8a 223.0 � 0.8c 156.5 � 18.1a 193.5 � 18.7b 229.0 � 16.5c 230.3 � 2.8c 226.2 � 8.4c 225.9 � 3.2c
140.7 � 32.2a 204.5 � 12.3cd 170.5 � 5.35abc 227.7 � 5.1d 205.7 � 10.9cd 158.9 � 10.7ab 192.5 � 19.5bcd 155.1 � 9.4ab 147.8 � 19.4a
43.90 � 1.7a ND
26.57 � 6.3a ND
35.75 � 4.5c ND
81.75 � 4.9e ND
39.00 � 2.9bc ND
53.55 � 5.6c ND
37.43 � 3.6c ND
71.45 � 1.6d ND
43.29 � 1.9ab
43.04 � 4.3b
Reported values correspond to the mean � standard deviation. Different letters within the same columns indicate significant differences (p < 0.05) among samples.
Fig. 4. Weight loss versus temperature curves by TGA of composites based on thermoplastic starch (TPS), with 0, 0.5 and 1.5 wt % of: a) cassava peel, b) ahipa peel, c) cassava bagasse, and d) ahipa bagasse. 5
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However, this effect was independent from the filler content (p > 0.05). Filler concentration on the other hand, had an impact on melting tem perature (Tm) and energy (ΔHm) of TPS with cassava by-products: higher filler content resulted in higher Tm though lower ΔHm. In general, however, an increase in melt enthalpy values of TPS composites was observed which could be indicative of an induction of crystallinity in the polymeric matrix due to the inclusion of fillers. The nucleating effect of filler agents have been proved on polymeric matrices such as PLA [49], although several studies on starch-based composites have shown different tendency depending on processing methods, filler type and content [8,44,50–52]. On TPS based films with filler contents lower than 10 wt % no significant effect on starch crystallization was reported [8, 44,51,52], yet further studies would be needed to confirm the main factors for the melt enthalpy variations herein reported. DMA results evidenced that, both glass transition temperatures (Tg1 and Tg2), espe cially the second transition corresponding to the starch-rich phase, were affected by filler type: TPS-CP > TPS-AP > TPS-CB > TPS-AB > TPS. This trend correlates with the filler’s fiber and lignin content (Table 1). 3.4. Optical, gas barrier and mechanical properties Color study revealed that films reinforced with cassava and ahipa peel presented the greatest differences with the control; being this effect proportional to the filler content (Fig. 5). These samples presented or ange shade in contrast to the more yellowish tone of TPS control (Fig. 5b). Color saturation and color differences were also affected by the filler type, though filler content had an insignificant or less important effect in TPS composites reinforced with cassava and ahipa bagasse (Fig. 5a and c). These results were attributed to the natural and char acteristic pigmentation of the fillers, especially of the peels. Moreover, the anthocyanins content of ahipa roots, relatively [53]vely high compared to cassava roots or other tubers [53], could have been partially retained in the ahipa bagasse particles and thus contributing to the higher color differences (Fig. 5). Since filler presence within TPS matrix can physically obstruct light, it would be expected that the incorporation of cassava and ahipa byproducts decreased films transparency and increased light blocking ef fect in all the spectrum (400–700 nm). However, the obtained results revealed that the transparency of the films was only affected with 1.5 wt % peel (Fig. 6). In correlation with color differences results, trans parency was more affected by cassava peel than ahipa one. It should be remarked though that, since the transparency parameter defined by Piermaria et al. [30] is proportional to the film absorbance at 600 nm, the higher this parameter the lower results the films transparency. As regards the charge blocking capacity it could be seen that within the visible range (750 nm) the tendency is the same as for the trans parency parameter (Fig. 6). Nonetheless, in the UV light range (350 and 300 nm) resulted to be quite independent of filler content (Fig. 6); this is reflected in the almost proportional decrease of the blocking parameter with filler content increase, meaning that the composites with 0.5 and 1.5 wt% of filler showed similar transmittance differences with de control. Among filler types, ahipa peel (AP) presented the higher UVlight blocking capacity, followed by cassava peel (CP), ahipa bagasse (AB) and cassava bagasse (CB), which correlates with the lignin content of the residues (Table 1). As shown in Table 3, all TPS composite films presented WVP values similar to those reported for other starch-based materials [54–56]. Even though the inclusion of fillers would be expected to increase tortuosity within the film matrix and therefore reduce water vapor diffusion, no significant differences (p > 0.05) were observed in WVP values for any composite regardless the filler type or content. These results are consistent with those reported previously for cassava starch films rein forced with cassava bagasse [9]. Mechanical behavior of TPS composites was significantly affected by filler type (Table 3). Cassava and ahipa bagasse incorporation gave place to more rigid and brittle materials, reflected in higher elastic moduli and
Fig. 5. Color parameters of composites based on thermoplastic starch (TPS), with 0, 0.5 and 1.5 wt % of cassava peel (CP), cassava bagasse (CB), ahipa peel (AP), and ahipa bagasse (AB): a) color saturation (Chroma*); b) color hue (Hue angle); and c) color difference (with a white standard). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
stress at break, and lower elongation capacity. On the contrary, cassava and ahipa peel reinforced TPS matrix, resulting in tougher materials with similar flexibility but higher strength (σm and E). Such effects were in all cases stressed by the filler content. 4. Conclusions The effect of low concentrations of organic fillers on thermoplastic corn starch properties was evaluated. As fillers it was employed bagasse and peel obtained during starch extraction from cassava and ahipa tu bers. Overall, results demonstrated a strong effect of filler type on TPS composite properties, even at low concentrations. In general, peel had a greater impact on thermal, mechanical, and optical properties of TPS films. However, thermal stability nor water vapor permeability of the samples were affected by the presence of studied fillers. In conclusion, it was demonstrated the feasibility of using tubers’ peel and bagasse as fillers of biodegradable polymer films in order to modify their properties 6
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Fig. 6. Light barrier capacity of composites based on thermoplastic starch (TPS), with 0, 0.5 and 1.5 wt % of cassava peel (CP), cassava bagasse (CB), ahipa peel (AP), and ahipa bagasse (AB): blocking effect at 750, 350 and 300 nm (Δ%T/%weight filler charge), and transparency (mm 1 � 102).
References
Table 3 Tensile mechanical resistance and water vapor permeability of composites based on thermoplastic starch (TPS) and cassava and ahipa by-products: peels (P) and bagasse (B). Composite
Mechanical properties P σm (MPa) b (%)
TPS
62.6 � 4.1bc 76.5 � 7.1d 64.3 � 9.9bc 61.7 � 4.5b 41.1 � 5.1a 73.2 � 8.6cd 73.5 � 6.5cd 55.7 � 8.8b 33.7 � 7.3a
TPS-CP0.5 TPS-CP1.5 TPS-CB0.5 TPS-CB1.5 TPS-AP0.5 TPS-AP1.5 TPS-AB0.5 TPS-AB1.5
E (Mpa)
WVP x 10 Pa)
10
1.19 � 0.04a 2.6 � 0.08b
22.7 � 1.7a
1.36 � 0.05a
78.2 � 9.7b
1.34 � 0.25a
3.0 � 0.15c
96.8 � 14.2b
1.30 � 0.01a
5.0 � 0.51d
192.3 � 31.2cd 217.9 � 13.4d 81.2 � 10.7b
1.45 � 0.20a
97.5 � 14.8b
1.41 � 0.10a
5.0 � 0.26d
182.0 � 30.2c
1.24 � 0.03a
5.1 � 0.51de
203.3 � 31.7
1.35 � 0.03a
5.5 � 0.41e 2.8 � 0.20bc 3.1 � 0.28c
cd
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1.56 � 0.31a 1.34 � 0.08a
Reported values correspond to the mean � standard deviation. Different letters within the same columns indicate significant differences (p < 0.05) among samples.
and to give added value to these agroindustry by-products. Acknowledgements �n This work was supported by the Agencia Nacional de Promocio �gica (ANPCyT, Project PICT 2011–1213, Científica y Tecnolo 2015–0921, 2014–2410) and the Consejo Nacional de Investigaciones Científicas y T� ecnicas (CONICET). Florencia Versino wishes to thank CONICET as well for a Doctoral and Postdoctoral Fellowship. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.compositesb.2019.107653.
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