Abstract
Bio-hybrid granules made from asphodel tuber (AT), starch, polyvinyl alcohol (PVOH), and dolomite were formed using a twin-screw extruder. The granules were prepared using starch or AT as a raw material, glycerol as a plasticizer, TiO2 as a heat stabilizer, and dolomite as a filler. The films were fabricated from granules by hot-press moulding. The mechanical (e.g., tensile strength, elongation-at-break, and modulus of elasticity), physical (e.g., weathered, density, hardness, color, water absorption and solubility in different temperatures, water vapor permeability, and oxygen permeability), and chemical properties (e.g., carbonyl index and vinyl index) of the films were analysed. The properties of the films were noticeably enhanced with AT and dolomite. Asphodel tuber improved the water solubility, water absorption, and weight loss after weathered. Asphodel tuber could resist water diffusion into the films because of its hydrophobic property, like dolomite. Dolomite also exhibited strong mechanical properties and barrier properties to water and oxygen. Additionally, cross-linking most likely occurred with inter- or intramolecular interactions. The interactions among the AT, starch, dolomite, and plasticiser with PVOH were interpreted as esterification, etherification, hydrogen bonding, carbonyl bonding, and vinyl bonding in the molecular structure of the bio-hybrid films.
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Effects of Asphodel Tuber and Dolomite on the Properties of Bio-hybrid Films Processed by a Twin Screw Extruder
Eyyüp Karaoğul *
Bio-hybrid granules made from asphodel tuber (AT), starch, polyvinyl alcohol (PVOH), and dolomite were formed using a twin-screw extruder. The granules were prepared using starch or AT as a raw material, glycerol as a plasticizer, TiO2 as a heat stabilizer, and dolomite as a filler. The films were fabricated from granules by hot-press moulding. The mechanical (e.g., tensile strength, elongation-at-break, and modulus of elasticity), physical (e.g., weathered, density, hardness, color, water absorption and solubility in different temperatures, water vapor permeability, and oxygen permeability), and chemical properties (e.g., carbonyl index and vinyl index) of the films were analysed. The properties of the films were noticeably enhanced with AT and dolomite. Asphodel tuber improved the water solubility, water absorption, and weight loss after weathered. Asphodel tuber could resist water diffusion into the films because of its hydrophobic property, like dolomite. Dolomite also exhibited strong mechanical properties and barrier properties to water and oxygen. Additionally, cross-linking most likely occurred with inter- or intramolecular interactions. The interactions among the AT, starch, dolomite, and plasticiser with PVOH were interpreted as esterification, etherification, hydrogen bonding, carbonyl bonding, and vinyl bonding in the molecular structure of the bio-hybrid films.
Keywords: Asphodel tuber; Dolomite; Bio-hybrid film; Polyvinyl alcohol; Twin-screw extruder
Contact information: Department of Food Engineering, Harran University, Sanliurfa, Turkey;
* Corresponding author: e.karaogul@harran.edu.tr
INTRODUCTION
The harmful effects of petroleum-based plastic packaging have increased public interest in the environment and biodegradable films (Wang et al. 2015). However, biodegradable polymers have limited industrial uses because of their poor mechanical and barrier properties. Thus, biopolymers have attracted increased research attention as alternatives to conventional non-degradable plastics (Karaoğul et al. 2018). In environmentally friendly polymers, raw materials are categorized as synthetic polymers (polyvinyl alcohol (PVOH), polylactic acid, etc.) and renewable natural polymers (starch, cellulose, chitosan, etc.) (Azahari et al. 2011). Starch is considered to be one of the most suitable materials among all-natural biopolymers because it is biodegradable, renewable, usually easily obtainable, and affordable (Lee et al. 2007). However, pure starch has numerous shortcomings, such as fragility, an un-plasticized characteristic, strong hydrophilic properties, and poor mechanical properties in the industry, which limit its use in common applications. Moreover, starch is not a thermoplastic because of intra- and intermolecular hydrogen bonds. (Luo et al. 2012). Therefore, the disadvantages of starch need to be improved. Plasticizers are used to improve the thermoplastic properties of starch. However, biodegradable films produced from starch are still limited because of their physicochemical properties, susceptibility to biological attack, and poor water resistance. Thus, PVOH as a synthetic biodegradable polymer and thermoplastic starch are used together and exhibit an excellent compatibility (Lum et al. 2013). Other natural polymers can be used instead of starch. Bio-hybrid films can be prepared by sol-gel (casting) and extrusion methods. However, because of the removal of remarkable amounts of water for aqueous solutions or suspensions, the casting method exhibits limitations, such as a high energy consumption, low solution density, low production, un-processability in the industry, and others (Thunwall et al.2008). Thus, moulding processes with extrusion are important for bio-hybrid films because it is energy efficient, has a high productivity, and is continuous on an industrial production level (Thunwall et al. 2008; Gao et al. 2012; Wang et al. 2015).
In this study, PVOH/starch granules were prepared by twin-screw extrusion. After this process, the granules were formed into films using hot-press moulding. Asphodel tuber (AT) (Asphodelus aestivus Brot.) was used as a natural polymer instead of pure starch because of its high starch and fibre contents (Karaoğul and Alma 2018). The main objective of this study was to characterize natural additive properties of AT and bio-plastic properties of AT/starch/PVOH bio-hybrid films with dolomite filler and low amounts of plasticizers.
EXPERIMENTAL
Materials
Asphodel tuber was collected from the geographical location with the latitude 37° 34’ 37.1” and longitude 36° 51’ 00.4” in a province located in the Mediterranean region of Turkey (2018). Starch (Part No. 0379) was purchased from TAT Nisasta Co. (Adana, Turkey). Polyvinyl alcohol (Code 088-20) with a viscosity of 17 cps, hydrolysis degree of 89%, and less than 1% ash content was provided by Birpa Kimya Co. (Ankara, Turkey). Glycerol was obtained with a purity of 99.5% from Tekkim Co. (Istanbul, Turkey). Titanium dioxide (TiO2) was obtained from Akdeniz Kimya Co. (Izmir, Turkey).
Experimental Design
The composition of the bio-hybrid films is shown in Table 1.
Table 1. Percent Composition of the PVOH/AT/Starch Blend Films
PVAF: PVOH film, SF: starch film, AF: AT film, SAF: starch and AT film, SDF: starch and dolomite film, ADF: AT and dolomite film
Six different formulations were prepared with various ratios of corn starch, AT, glycerol, TiO2, and dolomite, along with pure PVOH without starch, AT, or dolomite for comparison. The analyses data of PVAF and SF films were used as comparison references (Karaogul 2016) for AF, SAF, SDF, and ADF films. The starch or AT (40%), PVOH (40%), glycerol (14% of starch/ PVOH mass), TiO2 (3%), and water (3%) were mixed at 6000 rpm (600 W and 220 V) (G1 model mixer, Yazicilar Makina Co., Istanbul, Turkey) at room temperature for 5 min.
The prepared mixtures were first extruded with a twin-screw extruder (GM TWIN 25 Model, Gülnar Machinery Co., Istanbul, Turkey) at 170 °C in zone I, 165 °C in zone II, 160 °C in zone III, 155 °C in zones IV and V, and 160 °C in zones VI and VII of the barrel at a screw speed of 100 rpm with an L/D (Length per Diameter) ratio of 44. The extrudate was cut into pellets by granulation equipment (GM Pelletizer 2-6 Model, Gülnar Machinery Co., Istanbul, Turkey). Then, the pellets were moulded into the blend films, which had the dimensions 250 mm × 250 mm × 1 mm, using a hydraulic hot-press (4122CE Model, Carver, Wabash, USA) at 190 °C for 5 min. The produced hybrid films were then cooled to ambient temperature under a cold press. All of the test samples were prepared from the compression-moulded boards according to ASTM D638-14 (2014), as is shown in Fig. 1. The prepared samples were conditioned for at least 168 h at 23 °C ± 2 °C and a 65% ± 5% relative humidity in a climatic test chamber (Nüve, Ankara, Turkey).
Fig. 1. Bio-hybrid film dog-bone sample for mechanical test and a cutting die for preparing the samples.
Mechanical Characterisation
The mechanical characterisation of the hybrid films was investigated with an Alarge model analyser (TR, Istanbul, Turkey), according to ASTM D638-14 (2014). All of the tested films were 20 mm wide, 120 mm long, and had a 60-mm initial distance between the grips. The thickness of the films was evaluated using a digital stick at three random positions on the films. The analyser speed was 1 mm/s with six replicates for each test. The tensile strength (TS, MPa), elongation-at-break (E, %), and tensile modulus (TM, MPa) of the bio-hybrid films were determined.
Weathering Procedure
The bio-hybrid film samples were exposed to artificial ultraviolet light (λ = 360 nm; and light intensity = 38 W/m2) to accelerate weathered in a climate cabin. The climatic conditions were 40 °C and a 60% relative humidity. All of the samples were kept under the mentioned conditions for up to 340 h, and then the hardness, weight loss, color, and Fourier transform infrared (FT-IR) spectra (carbonyl index (CI) and vinyl index (VI)) were investigated. After that, all of the films were compared with the non-irradiated (reference) samples.
Water Vapour Permeability
The water vapour permeability (WVP) of the bio-hybrid films was examined using a gravimetric method according to ASTM E 96, E96M-16 (2016). The films were prepared with a radius of 12.3 mm and put between the lid and centrifuge tube of the permeability cell with three-quarters (approximately 10 mL) of its volume containing dried desiccant (CaCl2) (Fig. 2). Then, the centrifuge tube was placed in a climatic test cabin at 38 °C ± 2 °C and a 97% ± 1% relative humidity. The weight gains were measured at random 18 times over 244 h. In Fig. 3, all of the calibration curves were plotted based on linear regression analysis of the time (x, h) versus WVP (y, g) of the 18 marker constituents. The regression equation and correlation coefficient of the 18 markers were determined with linearity (R2 > 0.99) for all of the films with three replicates for each test.