International Journal of Biological Macromolecules 67 (2014) 458–462
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International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac
Preparation and characterization of bionanocomposite films based on potato starch/halloysite nanoclay Fatemeh Sadegh-Hassani, Abdorreza Mohammadi Nafchi ∗ Biopolymer Research Group, Food Science and Technology Division, Agriculture Department, Damghan Branch, Islamic Azad University, Damghan, Semanan, Iran
a r t i c l e
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Article history: Received 7 March 2014 Received in revised form 30 March 2014 Accepted 5 April 2014 Available online 18 April 2014 Keywords: Halloysite nanoclay, Starch film Mechanical properties
a b s t r a c t In this research casting method was used to prepare potato starch based bio-nanocomposite films with halloysite nanoclay as the reinforcing materials. The composition included potato starch with 40% (w/w) of a mixture of sorbitol/glycerol (weight ratio of 3 to 1as plasticizer) with nanoclay (0–5% w/w). The films were dried under controlled conditions. Physicochemical properties such as solubility in water, water absorption capacity (WAC), water vapour permeability (WVP), oxygen permeability, and mechanical properties of the films were measured. Results showed that by increasing the concentration of nanoclay, mechanical properties of films were improved. Tensile strength was increased from 7.33 to 9.82 MPa, and elongation at break decreased from 68.0 to 44.0%. Solubility in water decreased from 35 to 23%, and heat seal strength increased from 375 to 580 N/m. Also incorporation of clay nanoparticles in the structure of biopolymer decreased permeability of the gaseous molecules. In summary, addition of halloysite nanoclay, improve the barrier and mechanical properties of potato starch films and this bionanocomposites have high potential to be used for food packaging purposes. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Starch is found in abundance in nature and due to the low price, biodegradability, good mechanical and barrier properties, is one of the raw materials used for food packaging. Biopolymers exhibit high potential to be used as an alternative to plastics or synthetic polymers as they are more environmentally friendly and are easily biodegradable. The growth of biological products and the development of innovative technologies, reduce dependence on fossil fuels [1]. Sustainable and biodegradable packages are being developed worldwide and make great efforts to produce the packaging material of natural origin (proteins, fats and carbohydrates) to take films or coatings. Biopolymers have advantages over the synthetic polymers; biopolymers are biodegradable and renewable materials. Permeability inhibition of gases by biopolymers increases shelf life of fresh products such as fruits and vegetables [2]. Inhibitory to volatile compounds and oils maintain the quality of the food in production and distribution cycle. Besides, the film additives in the form of plasticizers (glycerol or sorbitol) used to enhance the
∗ Corresponding author. Tel.: +98 232 522 5045/+98 935 501 1872; fax: +98 232 522 5039. E-mail addresses: [email protected], [email protected] (A. Mohammadi Nafchi). http://dx.doi.org/10.1016/j.ijbiomac.2014.04.009 0141-8130/© 2014 Elsevier B.V. All rights reserved.
flexibility of biopolymer films are also reported to impair the barrier properties [3]. However, poor mechanical properties and high permeability to water vapour are two main disadvantages of biopolymers that recently nanotechnology helps to solve these problems. The use of nanotechnology in the field of polymer science has led to the production of nanocomposites polymers [4]. Bionanocomposites represent the new generation of nanocomposites, and comprises of the combination of biopolymers and an inorganic material that has at least one nanometer scale dimension. Bionanocomposites is an emerging group of nanostructured hybrid materials in the frontier between nanotechnology, material science, and life science [5,6]. Efforts have been devoted to the development of nanobiocomposites with enhanced thermal, mechanical, and functional properties due to the matrix or nanoparticle fillers [7,8]. In addition, biopolymer-based materials are known as a green technology and have shown biodegradability and biocompatibility in pharmaceutical, food packaging, and agriculture technologies [9,10]. Starch films stronger than other biopolymer films and mechanical properties of the starch films are better than polysaccharides and proteins film [11,12]. Mechanical properties of starch films are influenced by many factors such as amylose to amylopectin ratio in starch that plays an important role in the mechanical properties of the films. Starch films are transparent, semi-transparent, tasteless, colourless and flavourless. Starch because of its nature, cost
F. Sadegh-Hassani, A. Mohammadi Nafchi / International Journal of Biological Macromolecules 67 (2014) 458–462
and reliability of thermo plastics has a great interest as packaging materials. However, starch alone has some disadvantages such as hydrophilic nature and poor mechanical properties [13]. Potato starch has some number of desirable features to improve and enhance the stability and rigidity of the gel in some foods, and as a major component prefer to corn, wheat and other starches. The amount of phosphorus present in the amylopectin of potato starch has negative charge and repulsive little faster because there may be swelling of potato starch granules in hot water and high viscosity, high transparency and it is involved in low retrogradation speed of potato starch pastes [14]. Incorporation of nanoparticles such as nanosilver, titanium dioxide, nanoclay, nano zinc oxide, and silicon dioxide into food packaging materials is reported to exhibit improved mechanical, barrier, thermal, and physicochemical properties compared to the pure polymers or conventional composites [10,15–18]. A composite of starch and another additive might be a solution to improve these properties. Nano clay and nano silicon dioxide are examples of additive that is widely available, cost effective, and biodegradable and has been shown to improve the properties of various polymer materials [18]. Based on these observations, the major objective to undertake the present study was to develop a biodegradable starch-based nanocomposite film using halloysite nanoclay to enhance their functional properties. The method used in this research is casting method comprising the steps of gelatinization, pasting, bursting of granules, and starch retrogradation. It is envisaged that this type of films developed might find applicability and can be exclusively used in food packaging industry in the near future.
2. Materials and methods 2.1. Materials Starch from potato was purchased from Glucosan Company (Ghazvin, Iran). Glycerol and sorbitol was purchased from R&M Marketing (Essex, UK). Halloysite nanoclay (30 × 0.25–4 m, nanotube) was obtained from Sigma Chemical Co (St. Louis, MO, USA).
2.2. Film preparation Halloysite nanoclay was dispersed in water at different concentration (1, 2, 3, and 5% of total starch solid) and heated to 60 ◦ C with continuous stirring for 1 h, then sonicated in an ultrasonic bath (Marconi modelUnique USC 45 kHz) for 45 min to ensure homogenization was completed. The solution was cooled to room temperature and was used to prepare the aqueous starch dispersion 3.5% (w/w) [4]. The starch dispersion was heated to 85 ◦ C and held at this temperature for 45 min to complete the gelatinization. A 3:1 mixture of sorbitol–glycerol at 40% (w/w of starch) was added as plasticizers; the choice of the plasticizers was based on the previous research by Abdorreza et al. [3]. Upon completion of starch gelatinization, the solution was cooled to room temperature. A portion (90 g) of the dispersion was cast on Perspex plates fitted with rims around the edge to yield a 16 × 16 cm2 film-forming area. Films were dried under controlled conditions in a humidity chamber (25 ◦ C and 50% RH). Control films were prepared similarly but without addition of nanoparticles. Dried films were peeled and stored at 25 ± 2 ◦ C and 55 ± 5% relative humidity (RH) until experimentation. All films (including control) were prepared in duplicate. Film thickness was measured using a micrometer (Mitutoyo Co., Tokyo, Japan). Overall, 10 thickness measurements were taken for each film and the average values were recorded.
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2.3. Characterizations 2.3.1. Mechanical properties ASTM D882-10 [19] was used to determine the mechanical properties under standard conditions. Film strips were cut to 150 mm long and 25 mm wide and conditioned for 48 h in 25 ◦ C and 55% relative humidity. Texture analyzer (TA.XT2, Stable Micro System, Surrey, UK) equipped with Texture Exponent 32 software was used for measuring mechanical properties of the films. The initial grip separation and crosshead speed were 100 mm and 0.5 mm/s, respectively. Young’s modulus, elongation at break and tensile strength were calculated from the deformation and force data recorded by the software. Eight replicates for every sample were evaluated. 2.3.2. Heat seal strength measurement Heat sealability was determined with ASTM F88/F88M-09 [20] standard and with some modifications described by Mohammadi Nafchi et al. [21]. For each type of film, seal strength experiments were replicated five times in a randomized, complete block experiment. Film samples were cut into 7.62 × 2.5 cm strips. Two film strips were placed on top of one another, and an area of (25.4 × 20) mm2 (at the edge of the film) was heat-sealed using an impulse heat-sealer Model-SP-300H (Triumph Mercantile Corp, Taipei, Taiwan) at a temperature of 120 ± 5 ◦ C, a sealing time of 0.9 s and heat seal pressure approximately 1.8 × 105 Pa. Prior to determining seal strength all sealed film samples were conditioned at 23 ± 2 ◦ C and 50 ± 5% RH for 48 h with magnesium nitrate saturated solution. The distance between the clamps was 2.5 cm. A 30-kg static load cell and a test speed of 90 mm/min were used. Seal strength was calculated as follows: Seal strength =
peak force film width
(1)
The maximum force required to cause seal failure was reported as the seal strength in Newton/metre (N/m) [20]. 2.3.3. Solubility in water Solubility of the films in water was determined according to Nafchi et al. [17]. Pieces of film (2 × 3 cm ≈ 500 mg) were cut from each film and were stored in a desiccators with P2 O5 (0% RH) for 2 days. Samples were weighed to the nearest 0.0001 g and placed into beakers with 80 mL deionized water (18 M). The samples were stirred with constant agitation for 1 h at room temperature. The remaining pieces of film after soaking were filtered through filter paper (Whatman no.1), followed by oven drying at 60 ◦ C to constant weight. Samples were measured in triplicates. Water solubility was calculated as following equation: Solubility(%) =
(Initial dried weight of film − Final dried weight of film) × 100 Initial dried weight of film
(2)
2.3.4. Water absorption capacity The water absorption capacity measured as adapted method by Kiatkamjornwong et al. [22]. Nanobiocomposite films first were dried in a P2 O5 for one week and then a piece of 2 × 2 cm2 of dried films was added to 100 mL of distilled water and allowed to stand for 30 min for swelling. The swollen films were drained were weighted. The amount of water retained by films per dried weight of the films calculated as water absorption capacity. 2.3.5. Water vapour permeability (WVP) The modified gravimetric cup method [23] based on ASTM E9605 [24] was used to determine the water vapour permeability (WVP) of films. The test cups were filled with water 1.5 cm below the film. A plot of weight gained versus time was used to determine the WVTR. The slope of the linear portion of this plot represented
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Table 1 Mechanical properties of potato starch nanocomposite films. Nanoclay (%)
Tensile strength (MPa)
0 1 2 3 5
7.33 8.09 8.27 8.45 9.82
± ± ± ± ±
0.34c 0.49b 0.32b 0.25b 0.42a
Elongation at break (%) 68.0 61.5 54.5 52.4 44.0
± ± ± ± ±
1.8a 2.2b 6.1c 3.5c 3.9d
Young modulus (MPa) 188 297 315 332 376
± ± ± ± ±
14d 5c 7b 14b 12a
Values are mean (n = 12) ± SD. Different letters in each column represent significant difference at 5% level of probability among potato starch films.
the steady state amount of water vapour transmission through the film per unit time (g/h). Six samples per treatment were tested. The slopes yielded regression coefficients of 0.99 or greater. The WVP of film was calculated by multiplying the steady WVTR by the film thickness and dividing that by the water vapour pressure difference across the film.
2.3.6. Oxygen permeability (OP) Oxygen permeability measurements were performed on films with Mocon Oxtran 2/21 (Minneapolis, USA) equipped with a patented colometric sensor (Coulox® ) and WinPermTM permeability software. The meaurements were done using the ASTM standard method D3985-05. The films were placed on an aluminum foil mask with an open area of 5 cm2 and were mounted in diffusion cells. Tests were carried out at 25 ◦ C temperature, atmospheric pressure, and 50% RH using 21% oxygen as test gas. Transferred oxygen through the films was conducted by the carrier (N2 /H2 ) gas to the colometric sensor. Measurements were carried out on “convergent by hour” mode to reach the steady state of oxygen transmission. The permeability coefficients in cm3 m/(m2 day atm) were calculated on the basis of oxygen transmission rate in steady state taking into account the films thickness.
2.3.7. Hydrophobicity measurements To determine the hydrophobicity of the bionanocomposite films, water contact-angle measurements were performed on a static contact-angle meter (CAM-PLUS, Tantec, Germany). Data presented are the mean of eight independent determinations at different sites. Contact angles on each film surface were measured immediately after the addition of 1 L deionized water using the Sessile Drop Half-AngleTM Tangent line method [17].
2.3.8. Color measurement Using a colorimeter (Minolta CM-3500D; Minolta Co. Ltd., Osaka, Japan), we studied both transmission and reflection colorimetry of triplicate film samples. The instrument was calibrated with a zero transmittance calibration plate CM-A100 and air as full transmittance prior to use. A large size aperture was used. Color parameters (L* , a* , b* , C* , hab ) corresponding to the uniform color space CIELAB were obtained via the computerized system using Spectra Magic software version 2.11 (Minolta Cyberchrom Inc., Osaka, Japan).
2.4. Statistical analysis ANOVA and Tukey’s Post Hoc tests were used to compare means of physical, mechanical, thermal, and barrier properties of starch based films at the 5% significance level. Statistical analysis was conducted using IBMSPSS 21.0 for windows and GraphPad Prism 6 (GraphPad Software Inc., 2236 Avenida de la Playa, La Jolla, CA 92037, USA).
3. Results and discussions 3.1. Film formation and thickness In the present study, control potato starch films prepared were flexible, free standing, and transparent, while the freestanding potato starch/halloysite nanoclay nanocomposite films were milky, and opaque in appearance. An increase in the milky and opaque appearance was observed with a corresponding increase in the content of halloysite nanoclay. The mean thickness of the resulting nanocomposite films remained unchanged with the addition of nanoparticles. The average values of film thickness were 0.10–0.11 mm. 3.2. Mechanical properties Tensile strength (TS), elongation at break (E), and Young’s modulus (YM) of the bionanocomposite films are given in Table 1. As expected from other studies on biopolymer films reinforced by nanoparticles [2,25], significant increase was observed in TS and YM and a significant decrease was observed in elongation at break by addition of nanoclay. Increase in TS is of vital criteria for food packaging applications as high TS can allow the packaging films to withstand the normal stress encountered during food handling, shipping, and transportation. The main reason is related to the interfacial interaction between nanoclay and biopolymer matrix. The elasticity of the films is related to interactions of the macromolecules and can be reduced by addition of plasticizer. For elongation at break (E), a decreasing trend was found. Decrease in E can be desirable and advantageous for food packaging as it is directly related to the biodegradability of the films [18]. As shown in the previous researches nanoparticles significantly reduced moisture content [2]. It is likely that incorporation of nanoparticles occupy the sites on starch that normally would be occupied by water [25,26]. Mechanical properties of the composite films have been reported to be highly dependent on the interfacial interaction between the matrix and fillers [9]. Wu and others [27] showed that nanoparticles, used as a filling agent, improve the wear performance and tensile strength of starch films. The nanoparticles are likely to bond with hydroxyl groups and other possible hydrogen or Van der Walls bonds of starch macromolecules strengthening molecular forces between nanoparticles and starch [27]. 3.3. Heat seal strength Heat sealability is a critical factor in use of films for packaging purposes. Fig. 1 shows the effect of nanoclay on heat sealability of potato starch films. Potato starch films without nanoparticles had good heat sealability and the seal strength of near 500 N/m which is comparable with that of synthetic films (>600 N/m). Introduction of nanoclay improved heat sealability. These results support the theory proposed by Kim and Ustunol [28] using electron spectroscopy for chemical analysis (ESCA). They reported that increasing hydrogen and covalent bonds involving C O H and N C, which may be the main forces responsible for the sealed joint formation of
F. Sadegh-Hassani, A. Mohammadi Nafchi / International Journal of Biological Macromolecules 67 (2014) 458–462
Fig. 1. Effects of nanoclay on seal strength of potato starch films. Values are mean (n = 8) ± SD. Bars with different letters are significantly different at 5% level of probability.
the films. It was likely that Si O bonds interconnecting starch molecules improved the heat sealability. These results are consistent with data reported by Abdorreza and Karim [21]. 3.4. Solubility in water and water absorption capacity Solubility in water may be an important factor in defining applications for biopolymer composite films. Most of the biopolymers are sensitive to water. By incorporating lipids, crosslinking of the structure or incorporation of nanoparticles sensitivity to water could be decreased [29]. Fig. 2 shows the solubility of potato starch films in deionized water after 1 h. Incorporation of nanoparticles suppressed the diffusion of water into the structure. Effects of nanoclay on the water resistance of biopolymeric films reported by other researchers and these results are consistent with their results [18,27,30,31]. Water absorption capacity of the nanoclay biocomposite films are given in Fig. 3. Introducing nanoclay to potato starch matrix significantly decrease the water absorption capacity of the potato starch. This could be attributed to the interactions between clay and starch in film structure. The experimental results showed that when the nanoparticle content of potato starch films was increased, more hydrogen bonds formed between the SiO2 and the matrix
Fig. 2. Water solubility of potato starch films and effects of nanclay on solubility of films in water at 25 ◦ C. Values are means (n = 5) ± SD and bars with different letters are significantly different at 5% level of probability.
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Fig. 3. Water absorption capacity of potato starch films and effects of nanclay on WAC of films in water at 25 ◦ C. Values are means (n = 5) ± SD and bars with different letters are significantly different at 5% level of probability.
components [32]. For this reason, free water molecules do not interact as strongly as with nanocomposite films as with composite films alone. These results not only consistent with the water solubility results but also with results reported by other researchers on nanobiocomposites [2,32,33]. 3.5. Hydrophobic effects of halloysite nanoclay One method of comparing the hydrophobicity of material surfaces is the determination of the contact angle of a water droplet on the surfaces. A high contact angle indicates greater hydrophobicity of the surface and vice versa. The contact angle increased with increasing halloysite nanoclay content of potato starch films (Fig. 4), which indicates that the tendency to absorb water decreased and that the surfaces became more hydrophobic. These findings concur with the findings of water solubility and water absorption capacity investigations detailed above. 3.6. Water vapour permeability Decrease in the water vapour permeability is of high importance when developing food packaging materials where efficient barrier properties are desired to minimize moisture transfer between outside packaging environment and packaged food. Table 2 shows the effect of nanoparticles on the water vapour transmission rate through potato starch films. As expected, introduction of
Fig. 4. Water contact angle on potato starch films surfaces as a function of nanoclay content. The bars show mean (n = 8) ± SD. Different letters on the bars represent the significant difference at 5% level of probability.
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Table 2 Water vapour permeability, oxygen permeability, and tortuosity value of potato starch nanocomposites. Nanoclay (%)
WVP × 1011 [g/m/s/Pa]
0 1 2 3 5
2.91 2.14 1.44 1.02 0.83
± ± ± ± ±
O.P. [cm3 /m/(m2 day)]
0.53a 0.29b 0.24c 0.12d 0.11d
76.88 61.72 50.98 45.77 38.20
± ± ± ± ±
2.44a 1.28b 1.02c 0.50d 0.81e
O.P.composite /O.P.matrix
1 0.80 0.66 0.59 0.50
1 1.24 1.50 1.67 1.99
Values are mean (n = 5) ± SD. Different letters in each column represent significant difference at 5% level of probability among potato starch films. O.P.: Oxygen permeability coefficients at 25 ◦ C and 50% RH. : tortuosity value from Nielson’s equation.
nanoparticles significantly decreased water vapour permeability (WVP) of potato starch films. Nanoparticles filling the pores in the macromolecules structures can decrease gas permeability as well as permeability to water vapour [34]. Also other researchers have shown positive effects of nanoparticles on WVP. For example, Bajpai et al. [30] have shown that WVP of chitosan films added with zinc oxide nanoparticles decreased significantly. 3.7. Permeability to oxygen The results of oxygen permeability are presented in Table 2. Oxygen permeability coefficient significantly decreased from ∼70 to ∼45 cm3 m/(m2 day atm) by addition of halloysite nanoclay. The inorganic nanoclay particles are more water resistance than the biocomposite matrix; therefore incorporation of these nanoparticles to the matrix could likely introduced a tortuous pathway for oxygen molecules to pass through [35]. Based on the Nielsen’s [36] simple model on tortuosity, it is possible to describe the permeability reduction in halloysite nanoclay incorporated biocomposites. The reduction in permeability coefficients means that gases should travel in a longer diffusive path. The relation between tortuosity and permeability as given by Nielsen’s [36] model is 1 − s pc = pm
(3)
where is tortuosity, s is the volume fraction of halloysite nanoclay, Pm and Pc are the permeability coefficients of the biocomposite and bionanocomposite, respectively. Table 2 shows the tortuosity value for each bionanocomposite matrix. Zeppa et al. [37] and Nafchi et al. [2] also reported similar results for oxygen permeability by incorporation of nanoparticles. 3.8. Colour characteristics The color characteristic parameters for potato starch and nanoclay-incorporated potato starch nanocomposites are reported in Table 3. Overall, depending on the amount of nanoparticles added, the resulting films exhibited an increase in brightness and different color values. The lightness (L* ) significantly decreased with increasing halloysite nanoclay content. The transmissions of the films in different wavelengths significantly decreased upon addition of nanoparticles. This means that addition of nanoparticles to the films will improve the ability of the films to protect package contents from light and will enhance the quality of the packaged Table 3 Transparent colorimetric parameters of the starch films. Nanoclay (%)
L*
0 1 2 3 5
94.46 91.18 87.04 79.09 64.30
a* ± ± ± ± ±
0.71a 0.41b 0.40c 0.24d 0.23e
0.14 0.48 0.75 0.96 1.70
b* ± ± ± ± ±
0.07d 0.23c 0.16b 0.37b 0.08a
1.43 2.51 3.59 3.41 4.48
± ± ± ± ±
0.11d 0.20c 0.30b 0.14b 0.21a
Values are mean (n = 5) ± SD. Different letters in each column represent significant difference at 5% level of probability among potato starch films.
food. The same results were reported for the effects of nano SiO2 on potato starch nanocomposites [21]. 4. Conclusion In this study, we introduced halloysite nanoclay to potato starch matrix to fabricate bionanocomposites. Introduction of the nanoparticles improved mechanical properties of the films made from potato starches. Permeability to water vapour, and oxygen, water absorption capacity, and water solubility of the films significantly decreased. The results showed that under strict regulation, bionanocomposites based on halloysite nanoclay may have potential applications in food packaging industries. References [1] B. Ghanbarzadeh, H. Almasi, A.A. Entezami, Innovative Food Sci. Emerg. Technol. 11 (2010) 697–702. [2] A.M. Nafchi, R. Nassiri, S. Sheibani, F. Ariffin, A.A. Karim, Carbohydr. Polym. 96 (2013) 233–239. [3] M.N. Abdorreza, L.H. Cheng, A.A. Karim, Food Hydrocolloids 25 (2011) 56–60. [4] Z. Torabi, A. Mohammadi Nafchi, J. Chem. Health Risks 3 (2013) 33–42. [5] M. Darder, P. Aranda, E. Ruiz-Hitzky, Adv. Mater. 19 (2007) 1309–1319. [6] G.A. Ozin, A.C. Arsenault, L. Cademartiri, Nanochemistry: A Chemical Approach to Nanomaterials, Royal Society of Chemistry, Cambridge, 2009. [7] Y. Chen, S. Zhou, H. Yang, G. Gu, L. Wu, J. Colloid Interface Sci. 279 (2004) 370–378. [8] W. Jia, C. Liu, L. Yang, J. Yang, L. Fan, M. Huang, H. Zhang, G. Chao, Z. Qian, B. Kan, A. Huang, K. Lei, C. Gong, J. Zhao, J. Zhang, H. Deng, M. Tu, Y. Wei, Mater. Lett. 60 (2006) 3686–3692. [9] R. Yamaoki, T. Tsujino, S. Kimura, Y. Mino, M. Ohta, J. Nat. Med. 63 (2009) 28–31. [10] K. Shameli, M.B. Ahmad, W.M.Z.W. Yunus, Int. J. Nanomed. 5 (2010) 875–887. [11] R. Alebooyeh, A. Mohammadi Nafchi, M. Jokar, J. Chem. Health Risks 2 (2012) 13–16. [12] A. Mohammadi Nafchi, M. Moradpour, M. Saeidi, A.K. Alias, Starch—Stärke 65 (2013) 61–72. [13] J.H. Li, R.Y. Hong, M.Y. Li, H.Z. Li, Y. Zheng, J. Ding, Prog. Org. Coat. 64 (2009) 504–509. [14] R.P. Ellis, M.P. Cochrane, M.F.B. Dale, C.M. Duffus, A. Lynn, I.M. Morrison, R.D.M. Prentice, J.S. Swanston, S.A. Tiller, J. Sci. Food Agric. 77 (1998) 289–311. [15] X.H. Li, Y.G. Xing, W.L. Li, Y.H. Jiang, Y.L. Ding, Food Sci. Technol. Int. 16 (2010) 225–232. [16] K.S. Yao, D.Y. Wang, W.Y. Ho, J.J. Yan, K.C. Tzeng, Surf. Coat. Technol. 201 (2007) 6886–6888. [17] A.M. Nafchi, A.K. Alias, S. Mahmud, M. Robal, J. Food Eng. 113 (2012) 511–519. [18] H. Voon, R. Bhat, A. Easa, M.T. Liong, A.A. Karim, Food Bioprocess Technol. 5 (2012) 1766–1774. [19] ASTM, Annual Book of ASTM Standards, ASTM, Philadelphia, PA, 2010. [20] ASTM, Annual Book of ASTM Standards, ASTM, Philadelphia, PA, 2009. [21] A. Mohammadi Nafchi, A.A. Karim, J. Nano Res. 25 (2013) 90–100. [22] S. Kiatkamjornwong, W. Chomsaksakul, M. Sonsuk, Radiat. Phys. Chem. 59 (2000) 413–427. [23] T.H. McHugh, R. Avena-Bustillos, J. Krochta, J. Food Sci. 58 (1993) 899–903. [24] ASTM, Annual Book of ASTM Standards, ASTM, Philadelphia, PA, 2005. [25] D. Zou, H. Yoshida, J. Therm. Anal. Calorim. 99 (2010) 21–26. [26] D. Schlemmer, R.S. Angélica, M.J.A. Sales, Compos. Struct. 92 (2010) 2066–2070. [27] M. Wu, M. Wang, M. Ge, J. Text. Inst. 100 (2009) 254–259. [28] S.J. Kim, Z. Ustunol, J. Food Sci. 66 (2001) 985–990. [29] A. Pavlath, W. Orts, in: K.C. Huber, M.E. Embuscado (Eds.), Edible Films and Coatings for Food Applications, Springer, New York, NY, 2009, pp. 1–23. [30] S.K. Bajpai, N. Chand, V. Chaurasia, J. Appl. Polym. Sci. 115 (2010) 674–683. [31] M. Wu, Y. Wang, M. Wang, M. Ge, Fiber Text. East. Eur. 16 (2008) 96–99. [32] S. Tunc¸, O. Duman, Appl. Clay Sci. 48 (2010) 414–424. [33] C.M.O. Müller, J.B. Laurindo, F. Yamashita, Ind. Crops Prod. 33 (2011) 605–610. [34] X. Xia, Z. Hu, M. Marquez, J. Controlled Release 103 (2005) 21–30. [35] J. Yu, J. Yang, B. Liu, X. Ma, Bioresour. Technol. 100 (2009) 2832–2841. [36] L.E. Nielsen, J. Macromol. Sci. A: Chem. 1 (1967) 929–942. [37] C. Zeppa, F. Gouanvé, E. Espuche, J. Appl. Polym. Sci. 112 (2009) 2044–2056.
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International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac
Preparation and characterization of bionanocomposite films based on potato starch/halloysite nanoclay Fatemeh Sadegh-Hassani, Abdorreza Mohammadi Nafchi ∗ Biopolymer Research Group, Food Science and Technology Division, Agriculture Department, Damghan Branch, Islamic Azad University, Damghan, Semanan, Iran
a r t i c l e
i n f o
Article history: Received 7 March 2014 Received in revised form 30 March 2014 Accepted 5 April 2014 Available online 18 April 2014 Keywords: Halloysite nanoclay, Starch film Mechanical properties
a b s t r a c t In this research casting method was used to prepare potato starch based bio-nanocomposite films with halloysite nanoclay as the reinforcing materials. The composition included potato starch with 40% (w/w) of a mixture of sorbitol/glycerol (weight ratio of 3 to 1as plasticizer) with nanoclay (0–5% w/w). The films were dried under controlled conditions. Physicochemical properties such as solubility in water, water absorption capacity (WAC), water vapour permeability (WVP), oxygen permeability, and mechanical properties of the films were measured. Results showed that by increasing the concentration of nanoclay, mechanical properties of films were improved. Tensile strength was increased from 7.33 to 9.82 MPa, and elongation at break decreased from 68.0 to 44.0%. Solubility in water decreased from 35 to 23%, and heat seal strength increased from 375 to 580 N/m. Also incorporation of clay nanoparticles in the structure of biopolymer decreased permeability of the gaseous molecules. In summary, addition of halloysite nanoclay, improve the barrier and mechanical properties of potato starch films and this bionanocomposites have high potential to be used for food packaging purposes. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Starch is found in abundance in nature and due to the low price, biodegradability, good mechanical and barrier properties, is one of the raw materials used for food packaging. Biopolymers exhibit high potential to be used as an alternative to plastics or synthetic polymers as they are more environmentally friendly and are easily biodegradable. The growth of biological products and the development of innovative technologies, reduce dependence on fossil fuels [1]. Sustainable and biodegradable packages are being developed worldwide and make great efforts to produce the packaging material of natural origin (proteins, fats and carbohydrates) to take films or coatings. Biopolymers have advantages over the synthetic polymers; biopolymers are biodegradable and renewable materials. Permeability inhibition of gases by biopolymers increases shelf life of fresh products such as fruits and vegetables [2]. Inhibitory to volatile compounds and oils maintain the quality of the food in production and distribution cycle. Besides, the film additives in the form of plasticizers (glycerol or sorbitol) used to enhance the
∗ Corresponding author. Tel.: +98 232 522 5045/+98 935 501 1872; fax: +98 232 522 5039. E-mail addresses: [email protected], [email protected] (A. Mohammadi Nafchi). http://dx.doi.org/10.1016/j.ijbiomac.2014.04.009 0141-8130/© 2014 Elsevier B.V. All rights reserved.
flexibility of biopolymer films are also reported to impair the barrier properties [3]. However, poor mechanical properties and high permeability to water vapour are two main disadvantages of biopolymers that recently nanotechnology helps to solve these problems. The use of nanotechnology in the field of polymer science has led to the production of nanocomposites polymers [4]. Bionanocomposites represent the new generation of nanocomposites, and comprises of the combination of biopolymers and an inorganic material that has at least one nanometer scale dimension. Bionanocomposites is an emerging group of nanostructured hybrid materials in the frontier between nanotechnology, material science, and life science [5,6]. Efforts have been devoted to the development of nanobiocomposites with enhanced thermal, mechanical, and functional properties due to the matrix or nanoparticle fillers [7,8]. In addition, biopolymer-based materials are known as a green technology and have shown biodegradability and biocompatibility in pharmaceutical, food packaging, and agriculture technologies [9,10]. Starch films stronger than other biopolymer films and mechanical properties of the starch films are better than polysaccharides and proteins film [11,12]. Mechanical properties of starch films are influenced by many factors such as amylose to amylopectin ratio in starch that plays an important role in the mechanical properties of the films. Starch films are transparent, semi-transparent, tasteless, colourless and flavourless. Starch because of its nature, cost
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and reliability of thermo plastics has a great interest as packaging materials. However, starch alone has some disadvantages such as hydrophilic nature and poor mechanical properties [13]. Potato starch has some number of desirable features to improve and enhance the stability and rigidity of the gel in some foods, and as a major component prefer to corn, wheat and other starches. The amount of phosphorus present in the amylopectin of potato starch has negative charge and repulsive little faster because there may be swelling of potato starch granules in hot water and high viscosity, high transparency and it is involved in low retrogradation speed of potato starch pastes [14]. Incorporation of nanoparticles such as nanosilver, titanium dioxide, nanoclay, nano zinc oxide, and silicon dioxide into food packaging materials is reported to exhibit improved mechanical, barrier, thermal, and physicochemical properties compared to the pure polymers or conventional composites [10,15–18]. A composite of starch and another additive might be a solution to improve these properties. Nano clay and nano silicon dioxide are examples of additive that is widely available, cost effective, and biodegradable and has been shown to improve the properties of various polymer materials [18]. Based on these observations, the major objective to undertake the present study was to develop a biodegradable starch-based nanocomposite film using halloysite nanoclay to enhance their functional properties. The method used in this research is casting method comprising the steps of gelatinization, pasting, bursting of granules, and starch retrogradation. It is envisaged that this type of films developed might find applicability and can be exclusively used in food packaging industry in the near future.
2. Materials and methods 2.1. Materials Starch from potato was purchased from Glucosan Company (Ghazvin, Iran). Glycerol and sorbitol was purchased from R&M Marketing (Essex, UK). Halloysite nanoclay (30 × 0.25–4 m, nanotube) was obtained from Sigma Chemical Co (St. Louis, MO, USA).
2.2. Film preparation Halloysite nanoclay was dispersed in water at different concentration (1, 2, 3, and 5% of total starch solid) and heated to 60 ◦ C with continuous stirring for 1 h, then sonicated in an ultrasonic bath (Marconi modelUnique USC 45 kHz) for 45 min to ensure homogenization was completed. The solution was cooled to room temperature and was used to prepare the aqueous starch dispersion 3.5% (w/w) [4]. The starch dispersion was heated to 85 ◦ C and held at this temperature for 45 min to complete the gelatinization. A 3:1 mixture of sorbitol–glycerol at 40% (w/w of starch) was added as plasticizers; the choice of the plasticizers was based on the previous research by Abdorreza et al. [3]. Upon completion of starch gelatinization, the solution was cooled to room temperature. A portion (90 g) of the dispersion was cast on Perspex plates fitted with rims around the edge to yield a 16 × 16 cm2 film-forming area. Films were dried under controlled conditions in a humidity chamber (25 ◦ C and 50% RH). Control films were prepared similarly but without addition of nanoparticles. Dried films were peeled and stored at 25 ± 2 ◦ C and 55 ± 5% relative humidity (RH) until experimentation. All films (including control) were prepared in duplicate. Film thickness was measured using a micrometer (Mitutoyo Co., Tokyo, Japan). Overall, 10 thickness measurements were taken for each film and the average values were recorded.
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2.3. Characterizations 2.3.1. Mechanical properties ASTM D882-10 [19] was used to determine the mechanical properties under standard conditions. Film strips were cut to 150 mm long and 25 mm wide and conditioned for 48 h in 25 ◦ C and 55% relative humidity. Texture analyzer (TA.XT2, Stable Micro System, Surrey, UK) equipped with Texture Exponent 32 software was used for measuring mechanical properties of the films. The initial grip separation and crosshead speed were 100 mm and 0.5 mm/s, respectively. Young’s modulus, elongation at break and tensile strength were calculated from the deformation and force data recorded by the software. Eight replicates for every sample were evaluated. 2.3.2. Heat seal strength measurement Heat sealability was determined with ASTM F88/F88M-09 [20] standard and with some modifications described by Mohammadi Nafchi et al. [21]. For each type of film, seal strength experiments were replicated five times in a randomized, complete block experiment. Film samples were cut into 7.62 × 2.5 cm strips. Two film strips were placed on top of one another, and an area of (25.4 × 20) mm2 (at the edge of the film) was heat-sealed using an impulse heat-sealer Model-SP-300H (Triumph Mercantile Corp, Taipei, Taiwan) at a temperature of 120 ± 5 ◦ C, a sealing time of 0.9 s and heat seal pressure approximately 1.8 × 105 Pa. Prior to determining seal strength all sealed film samples were conditioned at 23 ± 2 ◦ C and 50 ± 5% RH for 48 h with magnesium nitrate saturated solution. The distance between the clamps was 2.5 cm. A 30-kg static load cell and a test speed of 90 mm/min were used. Seal strength was calculated as follows: Seal strength =
peak force film width
(1)
The maximum force required to cause seal failure was reported as the seal strength in Newton/metre (N/m) [20]. 2.3.3. Solubility in water Solubility of the films in water was determined according to Nafchi et al. [17]. Pieces of film (2 × 3 cm ≈ 500 mg) were cut from each film and were stored in a desiccators with P2 O5 (0% RH) for 2 days. Samples were weighed to the nearest 0.0001 g and placed into beakers with 80 mL deionized water (18 M). The samples were stirred with constant agitation for 1 h at room temperature. The remaining pieces of film after soaking were filtered through filter paper (Whatman no.1), followed by oven drying at 60 ◦ C to constant weight. Samples were measured in triplicates. Water solubility was calculated as following equation: Solubility(%) =
(Initial dried weight of film − Final dried weight of film) × 100 Initial dried weight of film
(2)
2.3.4. Water absorption capacity The water absorption capacity measured as adapted method by Kiatkamjornwong et al. [22]. Nanobiocomposite films first were dried in a P2 O5 for one week and then a piece of 2 × 2 cm2 of dried films was added to 100 mL of distilled water and allowed to stand for 30 min for swelling. The swollen films were drained were weighted. The amount of water retained by films per dried weight of the films calculated as water absorption capacity. 2.3.5. Water vapour permeability (WVP) The modified gravimetric cup method [23] based on ASTM E9605 [24] was used to determine the water vapour permeability (WVP) of films. The test cups were filled with water 1.5 cm below the film. A plot of weight gained versus time was used to determine the WVTR. The slope of the linear portion of this plot represented
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Table 1 Mechanical properties of potato starch nanocomposite films. Nanoclay (%)
Tensile strength (MPa)
0 1 2 3 5
7.33 8.09 8.27 8.45 9.82
± ± ± ± ±
0.34c 0.49b 0.32b 0.25b 0.42a
Elongation at break (%) 68.0 61.5 54.5 52.4 44.0
± ± ± ± ±
1.8a 2.2b 6.1c 3.5c 3.9d
Young modulus (MPa) 188 297 315 332 376
± ± ± ± ±
14d 5c 7b 14b 12a
Values are mean (n = 12) ± SD. Different letters in each column represent significant difference at 5% level of probability among potato starch films.
the steady state amount of water vapour transmission through the film per unit time (g/h). Six samples per treatment were tested. The slopes yielded regression coefficients of 0.99 or greater. The WVP of film was calculated by multiplying the steady WVTR by the film thickness and dividing that by the water vapour pressure difference across the film.
2.3.6. Oxygen permeability (OP) Oxygen permeability measurements were performed on films with Mocon Oxtran 2/21 (Minneapolis, USA) equipped with a patented colometric sensor (Coulox® ) and WinPermTM permeability software. The meaurements were done using the ASTM standard method D3985-05. The films were placed on an aluminum foil mask with an open area of 5 cm2 and were mounted in diffusion cells. Tests were carried out at 25 ◦ C temperature, atmospheric pressure, and 50% RH using 21% oxygen as test gas. Transferred oxygen through the films was conducted by the carrier (N2 /H2 ) gas to the colometric sensor. Measurements were carried out on “convergent by hour” mode to reach the steady state of oxygen transmission. The permeability coefficients in cm3 m/(m2 day atm) were calculated on the basis of oxygen transmission rate in steady state taking into account the films thickness.
2.3.7. Hydrophobicity measurements To determine the hydrophobicity of the bionanocomposite films, water contact-angle measurements were performed on a static contact-angle meter (CAM-PLUS, Tantec, Germany). Data presented are the mean of eight independent determinations at different sites. Contact angles on each film surface were measured immediately after the addition of 1 L deionized water using the Sessile Drop Half-AngleTM Tangent line method [17].
2.3.8. Color measurement Using a colorimeter (Minolta CM-3500D; Minolta Co. Ltd., Osaka, Japan), we studied both transmission and reflection colorimetry of triplicate film samples. The instrument was calibrated with a zero transmittance calibration plate CM-A100 and air as full transmittance prior to use. A large size aperture was used. Color parameters (L* , a* , b* , C* , hab ) corresponding to the uniform color space CIELAB were obtained via the computerized system using Spectra Magic software version 2.11 (Minolta Cyberchrom Inc., Osaka, Japan).
2.4. Statistical analysis ANOVA and Tukey’s Post Hoc tests were used to compare means of physical, mechanical, thermal, and barrier properties of starch based films at the 5% significance level. Statistical analysis was conducted using IBMSPSS 21.0 for windows and GraphPad Prism 6 (GraphPad Software Inc., 2236 Avenida de la Playa, La Jolla, CA 92037, USA).
3. Results and discussions 3.1. Film formation and thickness In the present study, control potato starch films prepared were flexible, free standing, and transparent, while the freestanding potato starch/halloysite nanoclay nanocomposite films were milky, and opaque in appearance. An increase in the milky and opaque appearance was observed with a corresponding increase in the content of halloysite nanoclay. The mean thickness of the resulting nanocomposite films remained unchanged with the addition of nanoparticles. The average values of film thickness were 0.10–0.11 mm. 3.2. Mechanical properties Tensile strength (TS), elongation at break (E), and Young’s modulus (YM) of the bionanocomposite films are given in Table 1. As expected from other studies on biopolymer films reinforced by nanoparticles [2,25], significant increase was observed in TS and YM and a significant decrease was observed in elongation at break by addition of nanoclay. Increase in TS is of vital criteria for food packaging applications as high TS can allow the packaging films to withstand the normal stress encountered during food handling, shipping, and transportation. The main reason is related to the interfacial interaction between nanoclay and biopolymer matrix. The elasticity of the films is related to interactions of the macromolecules and can be reduced by addition of plasticizer. For elongation at break (E), a decreasing trend was found. Decrease in E can be desirable and advantageous for food packaging as it is directly related to the biodegradability of the films [18]. As shown in the previous researches nanoparticles significantly reduced moisture content [2]. It is likely that incorporation of nanoparticles occupy the sites on starch that normally would be occupied by water [25,26]. Mechanical properties of the composite films have been reported to be highly dependent on the interfacial interaction between the matrix and fillers [9]. Wu and others [27] showed that nanoparticles, used as a filling agent, improve the wear performance and tensile strength of starch films. The nanoparticles are likely to bond with hydroxyl groups and other possible hydrogen or Van der Walls bonds of starch macromolecules strengthening molecular forces between nanoparticles and starch [27]. 3.3. Heat seal strength Heat sealability is a critical factor in use of films for packaging purposes. Fig. 1 shows the effect of nanoclay on heat sealability of potato starch films. Potato starch films without nanoparticles had good heat sealability and the seal strength of near 500 N/m which is comparable with that of synthetic films (>600 N/m). Introduction of nanoclay improved heat sealability. These results support the theory proposed by Kim and Ustunol [28] using electron spectroscopy for chemical analysis (ESCA). They reported that increasing hydrogen and covalent bonds involving C O H and N C, which may be the main forces responsible for the sealed joint formation of
F. Sadegh-Hassani, A. Mohammadi Nafchi / International Journal of Biological Macromolecules 67 (2014) 458–462
Fig. 1. Effects of nanoclay on seal strength of potato starch films. Values are mean (n = 8) ± SD. Bars with different letters are significantly different at 5% level of probability.
the films. It was likely that Si O bonds interconnecting starch molecules improved the heat sealability. These results are consistent with data reported by Abdorreza and Karim [21]. 3.4. Solubility in water and water absorption capacity Solubility in water may be an important factor in defining applications for biopolymer composite films. Most of the biopolymers are sensitive to water. By incorporating lipids, crosslinking of the structure or incorporation of nanoparticles sensitivity to water could be decreased [29]. Fig. 2 shows the solubility of potato starch films in deionized water after 1 h. Incorporation of nanoparticles suppressed the diffusion of water into the structure. Effects of nanoclay on the water resistance of biopolymeric films reported by other researchers and these results are consistent with their results [18,27,30,31]. Water absorption capacity of the nanoclay biocomposite films are given in Fig. 3. Introducing nanoclay to potato starch matrix significantly decrease the water absorption capacity of the potato starch. This could be attributed to the interactions between clay and starch in film structure. The experimental results showed that when the nanoparticle content of potato starch films was increased, more hydrogen bonds formed between the SiO2 and the matrix
Fig. 2. Water solubility of potato starch films and effects of nanclay on solubility of films in water at 25 ◦ C. Values are means (n = 5) ± SD and bars with different letters are significantly different at 5% level of probability.
461
Fig. 3. Water absorption capacity of potato starch films and effects of nanclay on WAC of films in water at 25 ◦ C. Values are means (n = 5) ± SD and bars with different letters are significantly different at 5% level of probability.
components [32]. For this reason, free water molecules do not interact as strongly as with nanocomposite films as with composite films alone. These results not only consistent with the water solubility results but also with results reported by other researchers on nanobiocomposites [2,32,33]. 3.5. Hydrophobic effects of halloysite nanoclay One method of comparing the hydrophobicity of material surfaces is the determination of the contact angle of a water droplet on the surfaces. A high contact angle indicates greater hydrophobicity of the surface and vice versa. The contact angle increased with increasing halloysite nanoclay content of potato starch films (Fig. 4), which indicates that the tendency to absorb water decreased and that the surfaces became more hydrophobic. These findings concur with the findings of water solubility and water absorption capacity investigations detailed above. 3.6. Water vapour permeability Decrease in the water vapour permeability is of high importance when developing food packaging materials where efficient barrier properties are desired to minimize moisture transfer between outside packaging environment and packaged food. Table 2 shows the effect of nanoparticles on the water vapour transmission rate through potato starch films. As expected, introduction of
Fig. 4. Water contact angle on potato starch films surfaces as a function of nanoclay content. The bars show mean (n = 8) ± SD. Different letters on the bars represent the significant difference at 5% level of probability.
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Table 2 Water vapour permeability, oxygen permeability, and tortuosity value of potato starch nanocomposites. Nanoclay (%)
WVP × 1011 [g/m/s/Pa]
0 1 2 3 5
2.91 2.14 1.44 1.02 0.83
± ± ± ± ±
O.P. [cm3 /m/(m2 day)]
0.53a 0.29b 0.24c 0.12d 0.11d
76.88 61.72 50.98 45.77 38.20
± ± ± ± ±
2.44a 1.28b 1.02c 0.50d 0.81e
O.P.composite /O.P.matrix
1 0.80 0.66 0.59 0.50
1 1.24 1.50 1.67 1.99
Values are mean (n = 5) ± SD. Different letters in each column represent significant difference at 5% level of probability among potato starch films. O.P.: Oxygen permeability coefficients at 25 ◦ C and 50% RH. : tortuosity value from Nielson’s equation.
nanoparticles significantly decreased water vapour permeability (WVP) of potato starch films. Nanoparticles filling the pores in the macromolecules structures can decrease gas permeability as well as permeability to water vapour [34]. Also other researchers have shown positive effects of nanoparticles on WVP. For example, Bajpai et al. [30] have shown that WVP of chitosan films added with zinc oxide nanoparticles decreased significantly. 3.7. Permeability to oxygen The results of oxygen permeability are presented in Table 2. Oxygen permeability coefficient significantly decreased from ∼70 to ∼45 cm3 m/(m2 day atm) by addition of halloysite nanoclay. The inorganic nanoclay particles are more water resistance than the biocomposite matrix; therefore incorporation of these nanoparticles to the matrix could likely introduced a tortuous pathway for oxygen molecules to pass through [35]. Based on the Nielsen’s [36] simple model on tortuosity, it is possible to describe the permeability reduction in halloysite nanoclay incorporated biocomposites. The reduction in permeability coefficients means that gases should travel in a longer diffusive path. The relation between tortuosity and permeability as given by Nielsen’s [36] model is 1 − s pc = pm
(3)
where is tortuosity, s is the volume fraction of halloysite nanoclay, Pm and Pc are the permeability coefficients of the biocomposite and bionanocomposite, respectively. Table 2 shows the tortuosity value for each bionanocomposite matrix. Zeppa et al. [37] and Nafchi et al. [2] also reported similar results for oxygen permeability by incorporation of nanoparticles. 3.8. Colour characteristics The color characteristic parameters for potato starch and nanoclay-incorporated potato starch nanocomposites are reported in Table 3. Overall, depending on the amount of nanoparticles added, the resulting films exhibited an increase in brightness and different color values. The lightness (L* ) significantly decreased with increasing halloysite nanoclay content. The transmissions of the films in different wavelengths significantly decreased upon addition of nanoparticles. This means that addition of nanoparticles to the films will improve the ability of the films to protect package contents from light and will enhance the quality of the packaged Table 3 Transparent colorimetric parameters of the starch films. Nanoclay (%)
L*
0 1 2 3 5
94.46 91.18 87.04 79.09 64.30
a* ± ± ± ± ±
0.71a 0.41b 0.40c 0.24d 0.23e
0.14 0.48 0.75 0.96 1.70
b* ± ± ± ± ±
0.07d 0.23c 0.16b 0.37b 0.08a
1.43 2.51 3.59 3.41 4.48
± ± ± ± ±
0.11d 0.20c 0.30b 0.14b 0.21a
Values are mean (n = 5) ± SD. Different letters in each column represent significant difference at 5% level of probability among potato starch films.
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