Food Sci. Biotechnol. 24(5): 1679-1685 (2015) DOI 10.1007/s10068-015-0218-5
Effects of Preparation Method on Properties of Poly(butylene adipate-co-terephthalate) Films Gaobin Li, Shiv Shankar, Jong-Whan Rhim*, and Bong-Yun Oh1 Department of Food Engineering and Bionanocomposite Research Institute, Mokpo National University, Muan, Jeonnam 58554, Korea 1 Food and Farm Management Research Institute, Jellanamdo Agricultural Research and Extension Services, Naju, Jeonnam 58213, Korea Received January 12 2015 Revised May 14 2015 Accepted June 18 2015 Published online October 31 2015 *Corresponding Author Tel: +82 61 450 2423 Fax: +82 61 454 1521 E-mail:
[email protected] pISSN 1226-7708 eISSN 2092-6456 © KoSFoST and Springer 2015
Abstract Poly(butylene adipate-co-terephthalate) (PBAT) films were prepared using 4 different preparation methods and the effects of the processing method on physical, mechanical, and structural properties of PBAT films were studied. Films were characterized using UV-visible spectroscopy, FT-IR, XRD, and thermogravimetric analysis. All films were white in color and exhibited high UV-barrier properties. Films prepared using solvent casting (SC) were less transparent than other films. No structural changed were observed among PBAT films but films prepared at high temperatures exhibited a higher degree of crystallinity. Extrusion-cast (EC) films exhibited highest tensile strength values, while solvent-cast (SC) films had the lowest TS values. The water vapor permeability and water contact angle were not affected by the processing method. The degree of crystallinity of EC films was higher than for other films. Keywords: PBAT film, processing method, solvent casting, thermocompression, extrusion casting
Introduction Global annual production of plastics exceeds 250 million tons, and more than 40% of the total plastic production is used as packaging materials, of which approximately 50% is used in food packaging industries (1). The most widely used plastic packaging materials include polyethylene (PE), polypropylene (PP), polystyrene (PS), poly(vinylchloride) (PVC), poly(ethylene terephthalate) (PET), and nylon (2). However, petrochemical-based conventional packaging materials are non-biodegradable and cause serious environmental problems. Hence, there is an urgent need for development of biodegradable food packaging materials as alternatives to synthetic plastic packaging materials, especially for use in short-term packaging and disposable applications (1). Renewed interest for development of biodegradable packaging materials using biopolymers derived from renewable resources is in evidence. However, applications of biodegradable materials originating from natural biopolymers are limited because of poor mechanical, gas barrier, and processing properties (3). Recently, the synthetic biopolymers poly(lactide) (PLA), poly(butylene adipate-co-terephthalate) (PBAT), poly(butylene succinate) (PBS), and other biodegradable polyesters have attracted attention for packaging and other value added applications due to environmently friendly natures, good processibility, and acceptable mechanical and barrier properties. PBAT is a promising candidate for use as a biodegradable packaging
material because of thermoplastic and fully biodegradable properties and a high elongation at break, low water vapor permeability, and good processing properties suitable for preparation of packaging films, compost bags, and agricultural mulching films (4). PBAT is an aliphatic-aromatic copolyester, mainly derived from the monomers 1, 4-butanediol, adipic acid, and terephthalic acid with a tunable balance of butylene adipate and butylene terephthalate (5). PBAT has been reported to be degraded in a few weeks by lipases from Pseudomona cepacia and Candida cylindracea (6). PBAT films can be prepared using extrusion-casting, extrusion blowing, thermocompression, and solvent casting methods (7). Extrusion of PBAT is the preferred method for industrial production of films and thermocompression is also a simple and convenient method for production of films without use of a solvent. PBAT is soluble in chloroform and PBAT films can be prepared using a solvent casting method. Performance properties of polyester films are known to be greatly influenced by processing methods (8,9). Therefore, the main objective of this study was to test the effects of PBAT film preparation methods on film properties. PBAT films were prepared using solvent-casting (SC), thermocompression (TC), solvent-casting and thermocompression (STC), and extrusion-casting (EC) methods, and resultant films were characterized using UV-visible spectroscopy, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). Optical characteristics, color, mechanical features, and water vapor
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barrier properties of films were also investigated.
Materials and Methods Materials PBAT (EnPol PBG7070; m.p. 125oC, specific gravity of 1.20-1.25) was obtained from S-EnPol Co. Ltd. (Wonju, Korea). PBAT pellets were dried under vacuum at 60oC for 24 h before use. Chloroform was procured from Daejung Chemicals & Metals Co., Ltd. (Siheung, Korea). Preparation of PBAT films PBAT films were prepared using SC, TC, STC, and EC methods. SC films were prepared based on dissolution of 4 g of PBAT in 100 mL of chloroform with constant stirring at room temperature (8). The completely solublized solution was cast evenly on a leveled Teflon film (Cole-Parmer Instrument Co., Chicago, IL, USA) coated glass plate (24×30 cm) framed on 4 sides, spread evenly, and dried at room temperature (22±2oC) for approximately 24 h. Completely dried films were peeled off glass plates. TC film was prepared using a Carver laboratory press (Hydraulic unit, model 3925; Carver, Inc., Wabash, IN, USA) (8). Approximately 4 g of PBAT was placed between 2 stainless steel plates (1 mm thick, 25.4×25.4 cm2) lined with Teflon cloth, then placed between the platens of pressing platforms and press heated to 140oC. A pressure of approximately 25,000 psi (172 MPa) was applied for 3 min at a constant temperature of 140oC. The resultant PBAT film was peeled from Teflon cloth layers after cooling in air. STC film was pepraed using thermocompressing of SC film to remove remaining solvent. EC film was prepared based on the T-die extrusion method using a single screw extruder (BAU L25D19; Bautek Co., Ltd., Uijeongbu, Korea) attached to a film T-die head (Bautek Co., Ltd.) and a simultaneous film take-off unit with a chill roll (Bautek Co., Ltd.). The temperature profile of the feeding zone was set to 140 and 140-150oC, and the temperature of the die was 150oC with a speed rotation speed of 40 rpm. Characterization of PBAT films All films were conditioned in a constant temperature humidity chamber (Model FX 1077; Jeiotech Co., Ltd., Ansan, Korea) set at 25oC with 50% relative humidity (RH) for at least 3 days before testing. Film thickness was measured using a handheld micrometer (Dial thickness gauge 7301; Mitutoyo, Tokyo, Japan) with a 0.01 mm accuracy. The surface color of films was measured using a Chroma meter (Konica Minolta, CR-400; Minolta, Tokyo, Japan) with a white color standard plate as a film background (L=97.75, a= −0.49, and b=1.96) (10). Hunter color (L, a, and b values) was determined based on an average of 5 readings for each film sample. The total color difference (∆E) was calculated as: 2
2
∆E = (∆L) + (∆a) + (∆b)
Food Sci. Biotechnol.
2
where ∆L, ∆a, and ∆b are differences between color values of a standard color plate and a film specimen, respectively. Surface color was measured in triplicate and average values were reported. Optical properties of films were determined based on measurement of light absorption spectra and the transparency of films. Each film was cut into a square piece (5x5 cm) and placed between 2 plates of the magnetic film holder of spectrophotometer. Transmission of visible and UV light through films was determined based on measurement of percentage transmission values at wavelengths of 660 (T660) and 280 nm (T280), respectively, using a spectrophotometer (Mecasys Optizen POP Series UV/Vis; Mecasys Co., Ltd., Seoul, Korea). FT-IR spectra of films were obtained using an attenuated total reflectance-Fourier transform infrared (AT-FTIR) spectrophotometer (Tensor 37 Spectrophotometer with OPUS 6.0 software; Bruker Optics Inc., Billerica, MA, USA) in a wavenumber range of 600-4,000 cm−1 operated with a resolution of 4 cm−1. A square piece of film (2.5x2.5 cm) was placed on a glass slide and analyzed using an X-ray diffractometer (PANalytical Xpert pro MRD diffractometer; PANalytical, Almelo, the Netherlands) for XRD analysis. Spectra were recorded using Cu Kα radiation (wavelength= 1.54056Å) and a nickel monochromator filtering wave at 40 kV and 30 mA. Diffraction patterns were obtained at diffraction angles between 2θ=5-40o with a scanning speed of 0.2o/min at room temperature. Thermal analysis of PBAT films was performed using a differential scanning calorimeter (DSC Q100; TA Instruments, New Castle, DE, USA) following the method of Rhim et al. (8). Approximately 5 mg of a film sample was sealed in an aluminum pan and heated from −50 to 300oC at a scanning rate of 10oC/min under a nitrogen flow rate of 50 mL/min. For each film, the glass-transition temperature (Tg), crystallization temperature (Tc), and melting temperature (Tm) were determined from the thermogram. Tm and Tc values were taken as peak values of respective endotherms, and the Tg value was taken as the midpoint of heat capacity changes. The transition temperatures and the enthalpy of crystallization (∆Hc) and enthalpy of fusion (∆Hm) values were calibrated using indium as a standard. The degree of crystallinity (χc) of PBAT films was determined as: χc=100×(∆Hm−∆Hc)/∆Hmc where ∆Hmc is the enthalpy of fusion of pure crystalline PBAT=114 J/g (5). Thermal stability of PBAT films was evaluated following the TGA method using a TGA analyzer (TGA; Hi-Res TGA 2950 Thermogravimetric Analyzer, TA Instruments). Approximetely 5 mg of film was heated in a standard aluminum cup from 30 to 600oC at a heating rate of 10oC/min under a nitrogen flow rate of 50 mL/min. Tensile properties Film samples were cut into rectangular strips (2.54x10 cm) using a precision double blade cutter (Model LB,02/A; Metrotec, S. A., San Sebastian, Spain). Tensile properties of films
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including tensile strength (TS), elongation at break (E), and the elastic modulus (EM) were evaluated using an Instron Universal Testing Machine (Model 5565; Instron Engineering Corp., Canton, MA, USA) following ASTM Method D 882-88. The machine was operated in tensile mode with an initial grip separation set at 50 mm and a crosshead speed of 50 mm/min. The TS (Pa) value was determined based on division of the maximum load (N) by the initial cross-section area (m2) of a film sample, and the E (%) value was determined based on division of the extension at break of the film by the initial grip separation (50 mm) multiplied by 100. The EM (MPa) value was determined from the slope of the linear portion of the stress-strain curve, which corresponds to stress divided by strain for a film. Ten replicates were taken for each film sample and average values were reported.
assure symmetry and a horizontal level (12). Five measurements were taken for each film sample and average values were reported. Statistical analysis Experimental data were analyzed using a oneway analysis of variance (ANOVA). Significant differences among treatments were defined at p
