Apple Peel and Carboxymethylcellulose-Based Nanocomposite Films Containing Different Nanoclays So-Hyang Shin, Sung-Jin Kim, Soo-Hyun Lee, Ki-Moon Park, and Jaejoon Han

Biodegradable packaging films were developed from polymeric blends of apple peel powder (APP) and carboxymethylcellulose (CMC), into which different nanoclays were incorporated to produce a nanocomposite film. After first estimating the barrier and mechanical properties of 4 different biopolymer films (CMC, methylcellulose, gelatin, and polylactide), CMC was chosen as the best film-forming solution. Three different nanoclays (Cloisite Na+ , 30B, and 20A) were subsequently dispersed in a CMC film solution to improve the barrier and physical properties of the final CMC nanocomposite films. The structures of the exfoliated CMC nanocomposite films were characterized using X-ray diffraction (XRD) to determine the most efficient nanoclay type, with Cloisite Na+ addition being found to demonstrate the greatest improvement in the barrier and mechanical properties of the film. Finally, the CMC and Cloisite Na+ solution were thoroughly blended with APP using a high-pressure homogenization (HPH) process to develop biopolymer nanocomposite films, which were then characterized using XRD and Fourier transform infrared spectroscopy. The HPH treatment significantly improved the film-forming ability by increasing the dispersity of APP in the CMC nanocomposites, as well as having various other effects on the physical properties. These nanocomposite films can be viewed as an alternative solution for the use of agricultural biomass in developing environmentally friendly packaging materials.

Abstract:

E: Food Engineering & Physical Properties

Keywords: apple peel, biopolymer film, carboxymethylcellulose, high-pressure homogenization, nanoclay

Cloisite Na+ nanoclay noticeably improved the barrier and elongation properties of a biopolymer film. High-pressure homogenization successfully blended apple peel powder with carboxymethylcellulose to develop a nanocomposite film. The apple peel and CMC-based nanocomposite films that were developed could be used as a novel biodegradable packaging material.

Practical Application:

Introduction Petrochemical-based plastics such as polyolefins and polyesters have been increasingly used as packaging materials because of their ease of processing and handling, low price, and excellent chemical resistance (Tharanathan 2003). Their nonbiodegradability has, however, caused many environmental problems related to their disposal (Kim and Rhee 2003). A number of studies have therefore concentrated on finding substitutes for conventional petrochemical-based plastics using low-cost biomass or biopolymers with similar properties (Mali and others 2005; Mariniello and others 2007) Apples (Malus pumila Mill.) are one of the most widely cultivated fruits in the world, being second only to bananas in terms of total production (FAOSTAT 2010). Large amounts of apple peel are generated as a by-product of manufacturing, such as in the production of applesauce, apple juice, or canned apples (Akter and others 2010); it is estimated that some 9000 tons of apple peel are

MS 20131175 Submitted 8/19/2013, Accepted 12/10/2013. Authors Shin, Kim and Park are with Dept. of Food Science and Biotechnology, Sungkyunkwan Univ., Suwon, 440-746, Korea. Authors Lee and Han are with Dept. of Food Bioscience and Technology, Korea Univ., Seoul, 136-701, Korea. Direct inquiries to author Han (E-mail: [email protected]).

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generated each year by the apple processing industry (Henr´ıquez and others 2010). Consequently, new means of utilizing apple peel as biomass have been sought, including its use for the production of biodegradable plastic films. However, preliminary studies have shown that a film made from apple peel alone exhibits poor physical properties, which would restrict its application. Hence, it is necessary to find suitable ways to reinforce the physical properties of an apple-peel film, with one such possibility being to produce a film from a blend of apple peel and other biopolymers. Among the various biopolymeric materials currently available, cellulose derivatives such as CMC and methylcellulose (MC) are well known for their good film-forming properties and clarity (Minami and others 2006). Protein-based films such as gelatin (GEL), soy protein isolate, and whey protein isolate also demonstrate good film-forming properties and have relatively low oxygen permeabilities (Miller and Krochta 1997). Finally, polylactide (PLA) is a linear aliphatic thermoplastic polyester, produced either by the polycondensation of lactic acid or the ring opening polymerization of lactide. This has strong mechanical and barrier properties, but is also expensive to produce (Engelberg and Kohn 1991). The commercial success of biopolymers as food packaging materials has therefore been limited either by their high cost, or their inadequate mechanical and barrier properties (Sablani and others 2009). R  C 2014 Institute of Food Technologists

doi: 10.1111/1750-3841.12356 Further reproduction without permission is prohibited

Apple peel & CMC-based nanocomposite film . . . Table 1–Color and thickness of biopolymer films. Color Biopolymer film

L

CMC MC GEL PLA

± ± 0.10b ± 0.15c ± 0.41a

96.56 96.82 95.84 97.84

Thickness

A 0.14b

−0.26 −0.29 −0.47 −0.04

E

B

± ± 0.02b ± 0.08c ± 0.02a 0.05b

3.01 2.31 5.93 2.29

± ± 0.03c ± 0.26a ± 0.10d 0.09b

1.63 0.99 4.55 0.76

± ± 0.09c ± 0.29d ± 0.06a 0.14b

(μm) 63 74 69 79

± ± ± ±

0.5c 1.0b 0.5b 0.6a

Within a column, different superscripts indicate significant differences (P ࣘ 0.05). CMC, carboxymethylcellulose; MC, methylcellulose; GEL, gelatin; PLA, polylactide.

Nanocomposite films offer a new alternative to enhance the physical properties of biopolymers. Such composites typically consist of a polymer matrix (as a continuous phase) and a filler (as a discontinuous phase; Matthews and Rawlings 1999); with nanocomposites being those in which the fillers used are smaller than 100 nm in at least one dimension (nanofiller; Schadler and others 2007). Nanocomposites can improve the barrier and mechanical properties, and increase heat resistance when com-

pared to unmodified biopolymers and conventional composites (Sorrentino and others 2007). The most well-known of the nanoparticle types are layered clays such as montmorillonite (MMT), hectorite, sapnotite, and laponite. These nanoclays are known to be highly effective and versatile fillers (Zeng and others 2005) as a result of their stacked structure of silicate sheets with a high aspect ratio and large surface areas. This structure, along with vander Waals gaps formed between the layers,

Figure 1–Water vapor permeability (A) and oxygen permeability (B) of biopolymer films. CMC, carboxymethylcellulose; MC, methylcellulose; GEL, gelatin; PLA, polylactide. a–d Different letters indicate significant differences (P ࣘ 0.05).

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results in a strong attractive force between the sheets (Ray and Okamoto 2003). High-pressure homogenization (HPH) can be beneficially applied to disaggregate biomass or biopolymer particles using a combination of pressure and shear force (Paquin 1999). When HPH is applied to a biopolymer film-forming solution, it can collapse the biopolymer networks and biomass particles, thus causing these materials to become miscible and the film-forming solution to become homogenized. The physical and barrier properties, as well as the appearance of the resulting blended films, may therefore be different after HPH treatment (Sablani and others 2009; Maresca and others 2011). This work was aimed at developing nanocomposite films based on incorporating different nanoclays (Cloisite Na+ , 30B and 20A) into polymeric blends of apple peel powder (APP) and car-

boxymethylcellulose (CMC). This research was performed in 3 stages as follows: (1) evaluation of 4 biopolymer types and selection of one based on mechanical and barrier properties; (2) improvement of the mechanical and barrier properties of the selected biopolymer by incorporating nanoclay particles; and (3) development of a novel nanocomposite film based on incorporating nanoclays into a blend of the selected biopolymer (CMC) and APP.

Materials and Methods Materials Fresh apples were obtained from the Hwanggeum apple farm, located in Andong, Korea. After washing and peeling, the apple peels (with an average thickness of 0.36 ± 0.10 mm) were dehydrated in a drying oven at 45 °C for 24 h. The dried peels were

Figure 2–Tensile strength (A) and elongation at break (B) of biopolymer films. CMC, carboxymethylcellulose; MC, methylcellulose; GEL, gelatin; PLA, polylactide. a–d Different letters indicate significant differences (P ࣘ 0.05).

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Apple peel & CMC-based nanocomposite film . . . Table 2–Optical properties and thickness of CMC nanocomposite films. Color Film formulation

L

CMC NaD 30BD 20AD NaE 30BE 20AE

± ± ± ± ± ± ±

96.56 94.32 96.57 96.56 96.57 96.48 96.34

A 0.13a 0.07c 0.11a 0.12a 0.15b 0.11ab 0.13a

−0.26 −0.02 −0.03 −0.03 −0.01 0.00 −0.01

± ± ± ± ± ± ±

E

b 0.04d 0.02b 0.02a 0.02b 0.01c 0.02ab 0.02b

3.01 3.89 3.33 3.33 3.57 3.44 3.31

± ± ± ± ± ± ±

0.08d 0.08a 0.15c 0.15c 0.12b 0.16bc 0.08c

1.77 3.97 2.00 2.00 2.33 2.13 1.98

± ± ± ± ± ± ±

0.12d 0.06a 0.18cd 0.18cd 0.16b 0.19c 0.13cd

Transparency (%) 90.30 90.20 78.28 82.08 73.05 82.97 81.10

± ± ± ± ± ± ±

0.45a 0.27a 0.31e 0.10c 0.37f 0.50b 0.37d

Thickness (μm) 63 62 65 65 59 64 65

± ± ± ± ± ± ±

0.5a 0.7a 0.7a 0.6a 0.5a 0.6a 0.7a

Within a column, different superscripts indicate significant differences (P ࣘ 0.05). CMC, carboxymethylcellulose; NaD, CMC/Cloisite Na+ in distilled water; 30BD, CMC/Cloisite 30B in distilled water; 20AD, CMC/Cloisite 20A in distilled water; NaE, CMC/Cloisite Na+ in ethanol; 30BE, CMC/Cloisite 30B in ethanol; 20AE, CMC/Cloisite 20A in ethanol.

cept that only distilled water was selected for dispersion medium because of its better miscibility with CMC. The APP/CMC suspension and nanoclay solution were mixed and pre-homogenized using a homogenizer (model SR30, mtop-Korea, Seoul, Korea) at 18,000 rpm for 10 min. Each mixture (APP0, APP1, APP1.5, or APP2) was then degassed under vacuum and introduced into a high-pressure homogenizing processor (M-110S, Microfluidics Intl. Co., Newton, Mass., U.S.A.) with a H10Z (100 μm) interaction chamber. Three HPH passes were run at 170 MPa, though this process was not applied to the CMC or NaDfilm solutions. Each prepared film-forming solution was then poured onto a petri dish or teflon plate (in the case of the PLA films) and spread Film preparation Simple biopolymer films were prepared by dissolving 1 g of evenly by manually rotating the dish or plate. The casting plates glycerol as a plasticizer in 100 mL of distilled water, and then with film solutions were then placed on a level bench top and adding 3 g of CMC or GEL. The pH of the GEL film solution dried under ambient conditions for at least 24 h. was adjusted to 10 with 1N NaOH; the solution was then shifted to a water bath set at 90 °C for 30 min. A MC film was made in Film conditioning and thickness the same manner as the CMC film, except that 66% (v/v) ethanol The dried films were carefully peeled off the dishes and plates, was used as a solvent. To prepare a PLA film, 3 g of PLA was and were kept at 25 °C and 50% relative humidity (RH) by placdissolved in 100 mL of chloroform without any plasticizer. These ing them in a thermo-hygrostat (Lab-Made011, Sejong Scientific 4 different biopolymer film solutions were then prepared for an Co., Bucheon, Korea) for 48 h or more before testing. The film additional casting process. thickness was measured using a digital micrometer (ID-C112X. Among the 4 biopolymers; CMC was used to prepare nanocom- Mitutoyo Co., Kawasaki, Japan) accurate to within 0.1 μm, with posites with relatively high barrier and mechanical properties, the film thickness used in calculating the barrier and mechanical based on previous experimental data. A 3 g sample of CMC and 1 properties of the films. g of glycerol were first dissolved in 80 mL of distilled water; while 150 mg of each nanoclay, including Cloisite Na+ (Na), Cloisite 30B (30B), and Cloisite 20A (20A), was dispersed into 20 mL of Optical properties The color values of the films were measured using a colorimeter distilled water (D) or ethanol (E). Two different solvents (D and (Minolta CR-400 Chroma Meter, Konica Minolta Sensing, Inc., E) were used to disperse nanoclays because each nanoclay had the different surface hydrophilicity or hydrophobicity. The nanoclay Osaka, Japan) calibrated against a standard white plate. Film specsolutions were stirred for 24 h using a magnetic stirrer without imens were placed on the surface of the standard white plate, and heating to completely swell the clay. The fully hydrated nanoclay the Hunter L (lightness), a (redness), and b (yellowness) color valsolutions were sonicated for 1 h using a sonicator with an output ues were measured. The total color difference (E) was calculated power of 260 W (Ultrasonik 57H, NEY Equipment Div., Yucaipa, as follows: Calif., U.S.A), and were then added to the CMC film solutions to  E = (L − L 0 )2 + (a − a 0 )2 + (b − b 0 )2 prepare various CMC nanocomposite solutions, including NaD (Cloisite Na+ in distilled water), 30BD (Cloisite 30B in distilled water), 20AD (Cloisite 20A in distilled water), NaE (Cloisite Na+ where L 0 , a 0 , and b 0 are the Hunter color values of the standard in ethanol), 30BE (Cloisite 30B in ethanol), and 20AE (Cloisite white plate (L 0 = 97.55, a 0 = 0.17, and b 0 = 1.60). Measurements 20A in ethanol). were taken as the average of 5 different locations on each sample, APP/CMC nanocomposites were prepared by adding mixtures 1 at the center and 4 around the perimeter. The color values for of different APP:CMC mass ratios (0:3 for APP0; 1:2 for APP1; each type of film were determined for 6 replicates. 1.5:1.5 APP1.5; 2:1 for APP2) to solutions containing 80 mL The film transparency (T660 ) was determined using a UV-Vis of distilled water and 1 g of glycerol. The 20 mL Cloisite Na+ spectrophotometer (UV mini-1640, Shimadzu, Kyoto, Japan) in solutions were also prepared using the aforementioned method ex- transmittance mode. A film strip (25.4 × 50 mm) was cut and

then finely pulverized using an electric grinder (Daesung Artron Co., Seoul, Korea), and those particles smaller than 149 μm were separated out to obtain the final APP by sieving the powder with a standard sieve (US Nr. 100). The biopolymers CMC, MC, and gelatin (GEL) were purchased from Sigma-Aldrich Co., Ltd. (St. Louis, Mo., U.S.A.) where as PLA was supplied by SR Technopack Co. (Cheonan, Korea). Glycerol was purchased from Daejung Chemicals and Metals Co. (Siheung, Korea). Three types of nanoclays; Cloisite Na+ , Cloisite 30B, and Cloisite 20A; were purchased from Southern Clay Products Inc. (Gonzales, Tex., U.S.A.).

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a-f

Apple peel & CMC-based nanocomposite film . . . mounted on a film holder, and the percent transmittance was ber adjusted to 25 °C and 50% RH. After 24 h, the cups were reweighed and the WVP was calculated according to the following measured at 660 nm. formula:

Water vapor permeability The water vapor permeability (WVP) was determined gravimetrically using a modification of the cup method described by Han and others (2008). The cups contained anhydrous CaCl2 (0% RH) and were cylindrical in shape; with an outside diameter of 8.7 cm, an inside diameter of 4.1 cm, and a depth of 2.1 cm. The lids were ring shaped, with an outside diameter of 8.7 cm and an inside diameter of 4.6 cm. A film sample was placed on the mouth of the cup and covered with the lid. The sealed cups were preweighed using an electronic scale with a precision of 0.0001 g, and were left in an environmental cham-

WVP (g. mm/m2 .day. kPa) =

W.x A.T.(P2 − P1 )

where W is the increase in the weight of the cup after 24 h (g), x is the average thickness of the film (mm), A is the area of the film exposed to the air (m2 ), T is the measurement time (1 day), and P2 − P1 is the difference in water vapor pressure between the inside and the outside of the cup (kPa). WVP measurements of each film sample were determined for 6 replicates.

E: Food Engineering & Physical Properties

Figure 3–Water vapor permeability (A) and oxygen permeability (B) of CMC nanocomposite films. CMC, carboxymethylcellulose; NaD, CMC/Cloisite Na+ in distilled water; 30BD, CMC/Cloisite 30B in distilled water; 20AD, CMC/Cloisite 20A in distilled water; NaE, CMC/Cloisite Na+ in ethanol; 30BE, CMC/Cloisite 30B in ethanol; 20AE, CMC/Cloisite 20A in ethanol. a–f Different letters indicate significant differences (P ࣘ 0.05).

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Apple peel & CMC-based nanocomposite film . . . determined for 6 replicates, with the OP then calculated by dividing the OTR by the oxygen pressure and multiplying by the film thickness.

Mechanical properties The tensile strength (TS) and percent elongation at break (%E) were measured according to ASTM standard method D882–91 (1995) using a texture analyzer (TAXT plus 50, Stable Micro Systems Ltd., Vienna, U.K.). The films were cut into test strips with dimensions of 25.4 × 100 mm, which were then placed between the grip heads of the machine. The initial grip separation and cross-head speed were set at 50 mm and 500 mm/min, respectively. After the strip broke, a force–distance curve was obtained and the TS and %E were calculated as follows: TS (MPa) =

F A

Figure 4–Tensile strength (A) and elongation at break (B) of CMC nanocomposite films. CMC, carboxymethylcellulose; NaD, CMC/Cloisite Na+ in distilled water; 30BD, CMC/Cloisite 30B in distilled water; 20AD, CMC/Cloisite 20A in distilled water; NaE, CMC/Cloisite Na+ in ethanol; 30BE, CMC/Cloisite 30B in ethanol; 20AE, CMC/Cloisite 20A in ethanol. a–d Different letters indicate significant differences (P ࣘ 0.05).

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Oxygen permeability The oxygen permeability (OP) was measured in accordance with ASTM standard method D3985, using an oxygen permeation analyzer (Ox-Tran model 2/21, Mocon Inc., Minneapolis, Minn., U.S.A.) at 23 °C and 0% RH. A diffusion cell was divided by the test film into 2 compartments, with a carrier gas (a mixture of 98% N2 and 2% H2 ) flowing in the inside chamber and a test gas (pure O2 ) flowing in the outside chamber. The film sample was clamped into the diffusion cell, and the cell was closed by tightening the screws. The inside chamber was filled with the carrier gas to remove residual O2 , with the test gas then being introduced into the outside chamber of the diffusion cell. Oxygen transmission through the film material occurred because of the O2 tension difference between the inside and outside chambers. The concentration of O2 in the inside chamber consequently increased with time, and its level was detected by means of a coulometric sensor. After 24 h, the oxygen transmission rate (OTR) was obtained from the instrument. The OTR of each film sample was

Apple peel & CMC-based nanocomposite film . . . where F is the peak force at failure (N), and A is the cross-sectional FT-IR spectrometer (Bruker IFS 66/S, Bruker Optics Co., Etarea of the film (mm2 ). tlingen, Germany). The spectra were recorded in absorbance units, with the wave number ranging from 4000 to 400 cm−1 . lb − lo A total of 32 scans were collected, at an average resolution of %E = × 100 4 cm−1 . lo

E: Food Engineering & Physical Properties

where l b is the measured elongation at breakage (mm), and l o is Statistical analysis the original length of the film placed between the grip heads (50 The mean values for various parameters were calculated and mm). At least 9 replicates were analyzed for each film type. compared by an analysis of variance, using a general linear models procedure within the SAS software package for Windows (version X-ray diffraction (XRD) 9.2). Differences between the means of the groups were deterXRD analysis was conducted to investigate the structures of mined using analysis of variance by Duncan’s multiple-range test. pristine nanoclays and nanoclays incorporated into film matrices. The statistical significance was identified at the 95% confidence The XRD spectra were recorded using an X-ray diffractometer level (P ࣘ 0.05). (D8 Advance, Bruker AXS Co., Karlsruhe, Germany) with Cu Kα radiation at a wavelength of 0.15406 nm. The voltage and Results and Discussion current used were 40 kV and 100 mA, respectively. All of the samples were scanned in a 2θ range of 2−20°, at a scanning rate First stage: Characterization of the biopolymeric films Optical properties. These are of major importance in food of 4°/min. The interlayer distance of the nanoclays was calculated packaging films as these properties directly influence consumer from the lattice plane diffraction peak using Bragg’s equation: acceptability (Chillo and others 2008). The CMC, MC, GEL, and PLA films were transparent in appearance with good flexibility; λ = 2d 001 sin θ and as can be seen in Table 1, there is no great difference in the L where λ is the wavelength of the X-ray radiation, d 001 is the spacing and a values, though the b value of the GEL film is slightly higher than the others. The GEL film also had the highest E value, between the platelets of clay, and θ is the angle of diffraction. which is a direct result of its relatively high b value. Barrier properties. The main characteristic required of Fourier transform infrared (FT-IR) spectroscopy biopolymer packaging films is an ability to retard moisture and The formation of chemical bonds between the film-forming oxygen transport between the ambient atmosphere and food. Genpolymer materials and the nanoclays was demonstrated using an erally, the WVP values of biopolymer films are higher than those of petrochemical-based films (McHugh and others 1993). However, this WVP value depends on various factors, such as the hydrophobicity and hydrophilicity of the biopolymer molecules, the ratio between the crystalline and amorphous zones, and the polymeric chain mobility (Garcia and others 2000). The WVP measurements of different biopolymer films are shown in Figure 1(A), which demonstrates a wide variation ranging from 3.38 to 22.16 g·mm/m2 ·day·kPa. The GEL film displayed the highest WVP of 22.16 ± 0.92 g·mm/m2 ·day·kPa, which was followed by CMC at 7.30 ± 0.42 g·mm/m2 ·day·kPa, MC at 5.02 ± 0.12 g·mm/m2 ·day·kPa, and PLA at 3.24 ± 0.23 g·mm/m2 ·day·kPa. The low value of WVP observed in the PLA film is a result of its hydrophobic nature. Oxygen is the principal factor that causes oxidation, which subsequently leads to changes in food such as color, odor, flavor, and nutrient degeneration. Food packaging films with an appropriate OP can therefore help to improve the quality and extend the shelflife of food (Sothornvit and Pitak 2007). The results of OP measurement are presented in Figure 1(B), with the values for CMC, MC, GEL, and PLA films found to be 0.35 ± 0.02, 44.00 ± 3.45, 0.22 ± 0.02, and 296.86 ± 72.29 cc·μm/m2 ·day·kPa, respectively. Although the highest OP value was observed in the PLA film, the CMC, MC, and GEL films are still considered outstanding OP barriers compared to petrochemical-based films (Krochta and Nisperos-Carriedo 1994). It is usually expected that any biopolymer film made from polysaccharides, including starches and cellulose derivatives like MC and CMC, will provide a good oxygen barrier because of their firmly linked chemical bonds (Wang and Figure 5–XRD patterns of nanoclays and CMC nanocomposite films. CMC, others 2011). Therefore, biopolymer films with low OP values carboxymethylcellulose; Na, Cloisite Na+ ; 30B, Cloisite 30B; 20A, Cloisite typically have compact molecular structures, fewer pore cracks, 20A; NaD, CMC/Cloisite Na+ in distilled water; 30BD, CMC/Cloisite and rigid hydrogen bonded networks; which combine to create 30B in distilled water; 20AD, CMC/Cloisite 20A in distilled water; NaE, CMC/Cloisite Na+ in ethanol; 30BE, CMC/Cloisite 30B in ethanol; 20AE, tortuous paths for oxygen to pass through. Generally speaking, the higher cohesive energy density and lower free volume among CMC/Cloisite 20A in ethanol. E348 Journal of Food Science r Vol. 79, Nr. 3, 2014

Apple peel & CMC-based nanocomposite film . . . Table 3–Optical properties and thickness of APP/CMC nanocomposite films. Color Film formulation

L

CMC NaD APP0 APP1 APP1.5 APP2.0

± ± ± ± ± ±

96.56 94.32 95.96 86.53 82.61 79.26

a 0.13a 0.07c 0.06b 0.59d 0.49e 0.86f

−0.26 −0.02 −0.04 1.66 3.07 4.18

± ± ± ± ± ±

E

b 0.04e 0.02b 0.01d 0.12c 0.16b 0.22a

3.01 3.89 3.73 17.23 22.19 27.36

± ± ± ± ± ±

0.08e 0.08a 0.15d 0.51c 0.37b 0.51a

1.60 3.84 2.50 19.03 25.45 31.70

± ± ± ± ± ±

Transparency (%) 0.13f 0.06e 0.15d 0.73c 0.54b 0.88a

90.30 90.20 90.08 29.86 14.63 12.50

± ± ± ± ± ±

0.45a 0.27a 0.02a 0.08b 0.12b 0.13b

Thickness (μm) 63 62 64 64 60 63

± ± ± ± ± ±

0.5a 0.7a 0.5a 0.4a 0.4a 0.4a

Within a column, different superscripts indicate significant differences (P ࣘ 0.05). CMC, carboxymethylcellulose; NaD, CMC/Cloisite Na+ in distilled water; APP0, 0 g APP and 3 g CMC with HPH treatment; APP1, 1 g APP and 2 g CMC with HPH treatment; APP1.5, 1.5 g APP and 1.5 g CMC with HPH treatment; APP2, 2 g APP and 1 g CMC with HPH treatment.

polymer chains is closely related to the gas barrier properties of a polymer film. The free volume, which explains the vacant space between the molecules in a polymer, is used for the gas transport throughout the film itself; whereas a higher cohesive energy density acts as an opening to polymer chains and allows oxygen permeation (Miller and Krochta 1997). Consequently, the CMC and MC films were considered appropriate for formulating film solutions with APP in this study, based on their barrier properties. Mechanical properties. The CMC and MC films showed comparable TS values of 40.51 ± 3.06 and 36.83 ± 6.31 MPa, respectively, even though CMC had a slightly higher (P ࣘ 0.05) TS than MC (Figure 2). The GEL film displayed the highest TS value of 44.5 ± 6.10 MPa, whereas PLA had the lowest at 26.5 ± 1.81 MPa; however, its %E value was the highest among the biopolymer films tested. To replace the petrochemical-based films that have been widely used till now, a biopolymer film needs to demonstrate similar physical TS and %E values. From previous experiments (data not published), the %E values for both polyethylene (PE) and polypropylene (PP) were over 400% and hence, the %E values of both petrochemical-based polymers are far too high to be compared with biopolymers. However, the TS of PE is 14.76 MPa, whereas that of PP is 26.96 MPa (data not published); both of which are lower than the TS values of most of the tested biopolymer films. The TS value of PLA is closest to

that of PP and PE; however, the TS values of other biopolymer films are all significantly higher (P ࣘ 0.05) than PLA. Thus, a CMC film, which exhibited an appropriate TS value and barrier properties, was chosen as the subject for further composite film formulation.

Second stage: Characterization of nanocomposite films Optical properties. Compared with a CMC only film (control), the color parameters of CMC nanocomposite films are changed only slightly in terms of appearance (Table 2). In this study, the lightness (L) of the nanocomposite films slightly decreased; whereas the b value increased in all the nanocomposite films, consequently increasing the E value. The changes in the Hunter L, a, b, and E values are all attributable to the type of nanoclay, which considerably influences the degree of dispersion in a film-forming solution (Rhim and others 2011). In addition, there were also variations in the transparency (T660 ) values of the CMC nanocomposite films, being observed to vary from 78.28 to 90.20% as a result of the varying miscibilities between the nanoclays and the CMC polymer matrix. The CMC film and CMC/Cloisite Na+ composite film dispersed in distilled water (NaD) were both fairly transparent, though the other CMC nanocomposite films were less transparent. Cloisite Na+ in distilled water therefore appears to be more compatible with CMC

Figure 6–Water vapor permeability of APP/CMC nanocomposite films. CMC, carboxymethylcellulose; NaD, CMC/Cloisite Na+ in distilled water; APP0, 0 g APP and 3 g CMC with HPH treatment; APP1, 1 g APP and 2 g CMC with HPH treatment; APP1.5, 1.5 g APP and 1.5 g CMC with HPH treatment; APP2, 2 g APP and 1 g CMC with HPH treatment. a–e Different letters indicate significant differences (P ࣘ 0.05).

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in comparison with other nanoclays. It has generally been proven that the transparency of a well-developed nanocomposite film is not significantly changed when the clay platelets are completely dispersed in the polymer matrix, as this clay would not obstruct the passage of light (Rhim and others 2009). The CMC and NaD films therefore show no difference in their transparency values (P > 0.05) as a result of their complete exfoliation. Barrier properties. The Nanoclays provided an effective barrier against water and oxygen in the CMC films, regardless of the type of nanoclay used, or whether the pre-treatment occurred in water or ethanol (Figure 3). The addition of nanoclays significantly (P ࣘ 0.05) decreased WVP compared to the CMC control film; with CMC having a WVP value of 7.30 ± 0.42 g·mm/m2 ·day·kPa, whereas the WVP values of the CMC nanocomposite films ranged from 2.09 to 4.95 g·mm/m2 ·day·kPa. The WVP values of the CMC films composited with Cloisite Na+

were significantly (P ࣘ 0.05) lower than those with Cloisite 20A or Cloisite 30B. Because natural MMT (Cloisite Na+ ) is hydrophilic, whereas organically modified nanoclays such as Cloisite 30B and Cloisite 20A are hydrophobic, this result is most likely attributable to the surface hydrophilicity or hydrophobicity of the clay used (Rhim and others 2011). Considering the mechanism of diffusion, the potential for enhancing the barrier performance of the CMC nanocomposite films can be attributed to the large aspect ratio and surface area of the clay (Thellen and others 2005). That is, when dispersed sufficiently in a medium, these platelets can potentially form a tortuous pathway that water molecules must follow to pass through the film (Garc´ıa and others 2007). The OP values varied widely across the various CMC nanocomposite films, ranging from 0.004 to 0.029 cc·μm/m2 ·day·kPa, but demonstrated a drastic decrease of more than 1100% when

Figure 7–Tensile strength (A) and elongation at break (B) of APP/CMC nanocomposite films. CMC, carboxymethylcellulose; NaD, CMC/Cloisite Na+ in distilled water; APP0, 0 g APP and 3 g CMC with HPH treatment; APP1, 1 g APP and 2 g CMC with HPH treatment; APP1.5: 1.5 g APP and 1.5 g CMC with HPH treatment; APP2, 2 g APP and 1 g CMC with HPH treatment. a–e Different letters indicate significant differences (P ࣘ 0.05).

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nanoclays were added. This implies that OP is closely related to the degree of dispersion of the nanoclay platelets in the CMC polymer matrix. The layered silicates could be regarded as an impermeable obstacle to the motion of oxygen molecules, and thus can greatly influence the gas transport properties (Frounchi and Dourbash 2009). The nanoclays also have the potential to combine with hydroxyl groups in CMC through hydrogen bonds, resulting in a denser polymer matrix. This would have the effect of decreasing the solubility of water or gas molecules in the polymer matrix, and thus improve the barrier properties (Guti´errez and others 2012). Mechanical properties. The TS value of the CMC only film was 40.51 ± 3.06 MPa; however the addition of NaD raised this by 23% to a value of 49.38 ± 4.88 MPa, the highest TS value among all the tested films (Figure 4A). In fact, the TS values of the other CMC nanocomposite films were decreased from that of a CMC only film; and so it would seem that the exfoliated NaD nanoclays are able to form chemical bonds with biopolymers like CMC (Ray and Okamoto 2003). In the case of the other groups, including 30BD, 20AD, NaE, 30BE, and 20AE; it is assumed that the nanoclays were either not fully exfoliated, or not evenly dispersed in the CMC. Therefore, they do not have the same degree of interaction with CMC, resulting in the lower (P ࣘ 0.05) observed TS values compared to pure CMC and NaD. The TS value of the NaE film was lower than that of the NaD film, even though the Cloisite Na+ nanoclay was well exfoliated in the ethanol (Figure 5). This is caused by CMC not being completely dissolved in ethanol, and thus the aqueous CMC solution cannot be homogeneously blended with a Cloisite Na+ ethanol solution for the final NaE film formation. The %E values were significantly (P ࣘ 0.05) increased by more than 400% in all of the CMC nanocomposite films (Figure 4B); however, even though the %E values of the NaD and NaE films were higher than that of a pure CMC film, they were nonetheless lower than the %E values of the other nanoclays. This may be attributed to the chemical bonding between CMC molecules and exfoliated nanoclay (Cloisite Na+ ) in the NaD and NaE films, with the nanoclay particle itself acting as a plasticizer and enhancing the flexibility of the CMC films; whether they are exfoliated or not.

These results correspond with those obtained by XRD analysis and transparency measurement of the films, which indicated the degree of nanoclay exfoliation in the CMC polymer matrices. XRD. The XRD patterns shown in Figure 5 were obtained from pristine CMC film; Cloisite Na+ ; Cloisite 30B; Cloisite 20A; and CMC films containing nanoclays dispersed in different solvents, such as distilled water and ethanol. From this, it can be seen that the pristine Cloisite Na+ , Cloisite 30B, and Cloisite 20A patterns all display intense diffraction peaks at 2θ angles of 7.25, 4.77, ˚ and 3.18°; which represent d001 values of 12.2, 18.5, and 27.8A, respectively. Meanwhile, the pristine CMC exhibits a diffraction peak at a 2θ angle of 13.26°, which correlates to a d001 value of ˚ Because the d001 values of the nanoclays are much higher 6.7A. than that of pristine CMC, the CMC would seem to be intercalated well into the nanoclay structure. Moreover, the diffraction peaks associated with pure CMC disappeared or significantly decreased in those CMC nanocomposites where the nanoclays were applied using sonication. The absence of these peaks was particularly notable in the NaD and NaE groups, which implies that the nanoclays were fully exfoliated and the CMC polymer chains were intercalated in the Cloisite Na+ layers (NaD and NaE). This has the effect of stabilizing the molecular interactions, and maintaining the exfoliated state of the nanoclay and CMC (Guti´errez and others 2012). According to film’s optical, barrier, and mechanical properties as well as degree of exfoliation, Cloisite Na+ was chosen to prepare APP/CMC nanocomposite films in third stage.

Third stage: Characterization of nanocomposite films based on APP and CMC Optical properties. The surfaces of the APP/CMC nanocomposite films treated by HPH were found to be smooth, whereas those films without HPH treatment were visually nonuniform and rough. The optical properties of the APP/CMC nanocomposite films changed with an increase in the ratio of APP (Table 3), with a significant (P ࣘ 0.05) increase in the darkness, redness, and yellowness of the films; and a subsequent decrease in transparency. APP particles were dispersed homogeneously in the film by HPH, thus obstructing the light passing through the film

Figure 8–XRD patterns of APP/CMC nanocomposite films. CMC, carboxymethylcellulose; NaD, CMC/Cloisite Na+ in distilled water; APP0, 0 g APP and 3 g CMC with HPH treatment; APP1, 1 g APP and 2 g CMC with HPH treatment; APP1.5, 1.5 g APP and 1.5 g CMC with HPH treatment; APP2, 2 g APP and 1 g CMC with HPH treatment.

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Apple peel & CMC-based nanocomposite film . . .

Apple peel & CMC-based nanocomposite film . . .

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and increasing its opacity. However, based on the color change in the NaD and APP0 films, the HPH treatment itself does not directly influence the film’s optical properties; as the only difference between these films was whether or not the CMC nanocomposite film solution was treated by HPH. Barrier properties. The WVP values of CMC nanocomposite films with different APP ratios are presented in Figure 6. This shows that the HPH treatment reduced the APP particle size and improved the film-forming ability of the APP/CMC nanocomposite solutions, but again did not affect the WVP value of the CMC nanocomposite films itself; based on the results of the NaD and APP0 films. However, WVP was significantly (P ࣘ 0.05) increased by the addition of APP to the CMC nanocomposite, regardless of whether HPH treatment was used. These APP particles were physically dispersed within the CMC film solution, which expanded the free volume in the CMC polymer matrix and increased WVP (P ࣘ 0.05). Moreover, the OP values of the CMC nanocomposite films increased markedly by more than 1000% when APP particles were added (data not shown). Thus, the addition of APP to CMC nanocomposites is less favorable to the film’s barrier properties, but neither the nanoclay nor the HPH treatment counteracted the film’s barrier degradation. Mechanical properties. The TS values of the APP/CMC nanocomposite films treated by HPH are presented in Figure 7(A). Compared with NaD (without HPH treatment), the TS value of the APP0 film with HPH decreased by 13%; which was caused not only by a decrease in the CMC particle size, but also by the disintegration of CMC molecules after homogenization at 170 MPa. It is possible that the molecular interactions and crosslinks in CMC might be regenerated by HPH treatment; subsequently resulting in changes to the polymer structure, compactness, and rigidity (Fu and others 2011). Because the APP particle size is larger than the CMC and nanoclay molecules, HPH treatment is necessary with APP-containing solutions to ensure their miscibility and to develop a homogeneous film. However, even though the HPH treatment helped with film formation when APP particles were

added to CMC nanocomposites, the addition of APP significantly (P ࣘ 0.05) decreased the TS and %E values of the films. Moreover, the HPH treatment counteracted the positive effect of the nanoclay on the film’s degree of elongation (Figure 7B). Consequently, if the HPH process is to be applied to biomass-based polymer films like APP/biopolymer nanocomposites, then its negative effect on the nanoclay and positive effect on the film-forming ability will need to be comprehensively considered. XRD. XRD was performed to compare the pristine CMC, CMC nanocomposite, and APP/CMC nanocomposite films; as well as to analyze the function of the HPH treatment on the APP/CMC nanocomposites (Figure 8). The XRD diffraction peaks of the APP/CMC nanocomposite films exhibited similar values regardless of the APP concentration; with characteristic 2θ angle values appearing at 12.62° in APP0, 13.08° in APP1, 12.94° in APP1.5, and 12.94° in APP2. These results are slightly lower than those obtained for pristine CMC, which had a 2θ angle value of 13.26°. Consequently, the presence of APP increases the interlayer spacing, as a result of shifts in the diffraction peaks to lower angle values. Moreover, the XRD intensity of the APP/CMC nanocomposite films is reduced to the point that it becomes hard to recognize specific peaks. The NaD film (CMC nanocomposite without HPH) demonstrated a notably different XRD pattern compared with the HPH-treated CMC nanocomposite films (APP0, APP1, APP1.5, APP2), with these differences likely to be associated with the HPH process. Almasi and others (2010) developed CMC nanocomposites to improve starch-based films, finding that there was no significant change in the XRD intensity but that there was a change in the 2θ angle value. In our study, it appears that the XRD peaks are either scattered or less diffracted as a result of HPH treatment, meaning that the crystalline structures of the CMC and nanoclays are broken down to innumerable amorphous nanosized particles. FT-IR. The FT-IR spectra of pristine CMC film, pristine Cloisite Na+ , CMC nanocomposite film (NaD), and APP/CMC nanocomposite films with HPH treatment (APP0, APP1, APP1.5,

Figure 9–FT-IR spectra of CMC, pristine Cloisite Na+ , and APP/CMC nanocomposite films. CMC, carboxymethylcellulose; Na, Cloisite Na+ ; NaD, CMC/Cloisite Na+ in distilled water; APP0, 0 g APP and 3 g CMC with HPH treatment; APP1, 1 g APP and 2 g CMC with HPH treatment; APP1.5, 1.5 g APP and 1.5 g CMC with HPH treatment; APP2, 2 g APP and 1 g CMC with HPH treatment.

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APP2) are presented in Figure 9. The CMC film displayed distinct peaks at a wave number of 1035 cm−1 as a result of the OH group in the glycerol used as a plasticizer; at 1596 cm−1 as a result of the C = O stretching vibration; and at 1322 and 1415 cm−1 , both of which are ascribed to OH bending in the CMC. The absorption bands between 1000 and 1200 cm−1 are characteristic of the C-O stretching in CMC. The peak of the pristine Cloisite Na+ at 1045 cm−1 was attributed to the major peak of Si-O, which is one of the nanoclay molecules. The peak of the NaD film at 1045 cm−1 disappeared when combined with CMC, as the pristine Cloisite Na+ was efficiently exfoliated after sonication treatment; thus the peaks were similar to those of CMC. The APP0 was homogenized under a high pressure of 170 MPa, and so the particle size was much smaller than that of NaD. The HPH treatment did not influence the major chemical bonds in the CMC nanocomposites when compared with the spectra of NaD and APP0. Although APP was added to the CMC nanocomposite solution in different ratios, no newly formed peaks were observed. It can therefore be said that the APP particles did not develop chemical bonds with the CMC nanocomposite films, but rather were physically dispersed within the CMC nanocomposite polymer networks.

Conclusions CMC presented the best film-forming properties when compared to the other biopolymers that were tested, with its barrier and physical characteristics being greatly improved by the formation of a Cloisite Na+ nanocomposite. Apple peel was successfully blended with CMC using HPH treatment to develop an environmentally friendly biopolymer film. The nanocomposites enhanced the physicochemical properties of the biopolymer films, whereas the HPH technology provided a good solution for the use of agricultural biomass and by-products in developing biodegradable packaging materials.

Acknowledgments This research was supported by a Natl. Research Foundation of Korea (NRF) grant, funded by the Korea government (MEST) (Nr. 2011–0014710), and also supported by a Korea University grant (Nr. K1325371). The authors wish to thank the Inst. of Biomedical Science & Food Safety and Korea Univ. Food Safety Hall for providing necessary equipment and facilities.

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Apple peel & CMC-based nanocomposite film . . .

Apple peel and carboxymethylcellulose-based nanocomposite films containing different nanoclays.

Biodegradable packaging films were developed from polymeric blends of apple peel powder (APP) and carboxymethylcellulose (CMC), into which different n...
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