Mechanical Physicial Properties of Chlorella-PVA based Bioplastic with Ultrasonic Homogenizer

Public demand for environmentally friendly packaging material especially in food industry is increasing. One of the many solutions invented for this problem is the development of biodegradable plastic. Biopolymer can be mixed with synthetic polymer to produce biodegradable films with properties suitable for varying applications. This study examines the mechanical physical properties of Chlorellapolyvinyl alcohol (PVA) based bioplastic by pre-treating the Chlorella powder with ultrasonic homogenizer. Variation of Chlorella concentration and temperature was done during the ultrasonication. Before being used as bioplastic base, pre-treated Chlorella with different concentrations were equated. Bioplastic films were then prepared with the pre-treated Chlorella powder and PVA using solvent casting method. Mechanical physical properties of the pre-treated Chlorella films then compared with non pretreated Chlorella film as control. Mechanical test shows the increasing of bioplastic tensile strength up to 15,3 kgf/cm 2 and elongation percentage up to 99,63%. Field emission scanning electron microscopy test shows the increasing of bioplastic homogenity and smoother surface with less pores. Fourier transform infrared analysis shows that there are crosslinkages between Chlorella and PVA. Thermal analysis by thermogravimetric analysis shows ultrasonication creates a more compact linkages. The performance of the film could suggest its potential as an eco-sustainable food packaging plastic material.


Introduction
Food packaging with sufficient mechanical strength and thermal stability is substantial in food products for food quality protection and safety. Majority of the food packaging nowadays are petroleum based plastic. Unfortunately, petroleum based plastic has chemical building blocks that might harm people and the environment, and its production and disposal contribute to an array of environmental problems [1]. Several precautions were developed such as the development of biodegradable plastic (bioplastic). Bioplastic is made from natural polymer materials (biopolymer) and is designed to accelerate the natural degradation of plastic by microorganisms. Generally, biopolymers are used to blend with synthetic polymers to improve bioplastic characteristics and extend its applications. It is desired to use other biomass as a source of bioplastic production that improves bioplastic characteristics and to avoid food source competition. On the other hand, microalgae have been atracted as the most promisingly renewable resource for bioplastic production because of their faster growth rate, higher photosynthetic efficiency and biopolymer source. Microalgae perform oxygenic photosynthesis and are capable of taking up a large amount of CO2 [2]. The cultivation of microalgae can be enchanced by alteration of illumination during growth [3]. However, microalgae has relatively thick cell wall and large particles [4]. Ultrasonic technologies have been used as pre-treatment methods in various industrial fields for decades [5]. It is used to reduce small particles in a liquid so that they become uniformly small and evenly distributed. In this work, microalgae pre-treatment and its conversion to bioplastic with addition of polyvinyl alcohol (PVA) as synthetic polymer were attempted. The effect of ultrasonication treatment on bioplastic characteristics such as elongation, tensile strength, thermal properties, Field emission scanning electron microscopy, and Fourier transform infrared (FTIR) was clarified.

Biomass Pre-treatment
Chlorella: distilled water (C/W) solutions were treated with ultrasonic homogenizer. Dry 2,8 g of Chlorella powder was added into distilled water and then mixed with magnetic stirrer followed by ultrasonic treatment using a homogenizer (Merk) 53 kHz for 40 minutes. The treatment was done by the extraction method of starch from microalgae as described by Asada et al [5]. The test design of composisiton of this pretreatment is presented in Table 1. The solutions were taken and added with distilled water until Chlorella: distilled water of each solutions equals 1:10 for usage in the next method.

Preparation of films
Chlorella/Polyvinyl alcohol (PVA) blend films were prepared using the solvent casting method as described by Wang et al. [1] with modification of solution temperature, and all the bioplastic have the same composition. First, 2,8 g of PVA was dissolved in 10 mL of distilled water at 80 o C while Chlorella solution was added with 2,11 g of glycerol and heated at 90 o C. Thereafter, 1 g of citric acid was added to the PVA solution at 80 o C alongside with the addition of Chlorella and glycerol solution and were stirred for 30 minutes. All the solutions were cast onto glass plate (15 cm x 15 cm) and were dried for 24 h at room temperature and peeled off from the plate to obtain a dried film.

Mechanical Properties
The mechanical properties of the films were analyzed by measuring the tensile strength (TS) and elongation at break (E) according to the standard ASTM method D882-12 using an Shimadzu Universal Testing Machine.

Thermal Properties
The thermal stability of film samples was evaluated according to the standard ASTM method D1131-14 using a Mettler Toledo thermogravimetric analyzer.

Surface Morphology and FTIR Analysis
The surfaces of the films were fixed with conductive adhesive and were sprayed with platina. The composite films were observed using a field JEOL JSM-6510 LA Scanning Electron Microscope. By using Spectrum 100 Fourier transform infrared spectroscopy (FT-IR), spectroscopy analysis was performed.

Mechanical Properties
Mechanical properties such as tensile strength and elongation of the Chlorella/PVA blend films are shown in Table 3. Tensile test result shows that C/W 1:5:25 doubles the tensile strength and elongation percentage of Chlorella based bioplastic film (control). Optimum composition of solvent in a sonication process depends on the cell particle size and thickness [6]. Asada et al. (2012) [5] research shows that microalgae concentration of 1 g/mL in sonication treatment resulting in starch extraction ratio of 93,8%. Optimum condition of sonication resulting in higher amount of cell disruption and more dispersed particles which allows water molecule to intervene the polymer chain thus resulting in more flexible bioplastic film. Temperature also takes a big role in this reaction [5]. The higher the temperature, the weaker polymer bond will be, thus the easier water molecule to intervene the polymer chain. The tensile strength properties peak is at the Chlorella:water ratio of 1:5 because the amount of polymer-polymer bonds is equivalent to the amount of polymer-plasticizer bonds, while ratio of 1:3 has a higher amount of polymerpolymer bonds than the polymer-plasticizer bonds, and the ratio of 1:10 has higher amount of polymerplasticizer bonds than the amount of polymer-polymer bonds. As a result, the optimum condition of Chlorella sonication is Chlorella:water ratio of 1:5 with the temperature of 25 o C.
The result of comparing Chlorella-PVA based bioplastic film with ultrasonic pre-treatment with Chlorella-PVA based bioplastic film without ultrasonic pre-treatment shows that ultrasonication increases the tensile strength of the film up to 1,8x and increases the elongation percentafe up to 2,5x. This shows that ultrasonication improves the mechanical characteristic of Chlorella-PVA based bioplastic film.

Thermal Properties
TGA thermograms of Chlorella based bioplastic film with ultrasonic pre-treatment shows that the initial thermal decomposition at 80 -140 o C was due to evaporation of water accounting for weight loss of 7,74% of the total mass of the sample, and then the main thermal decomposition was at 200 -440 o C due to chemical degradation by polymer chain scission accounting for weight loss of 75,4% of the total mass of the sample. Final residue after final heating of 600 o C of Chlorella-PVA based bioplastic film with ultrasonic pretreatment and Chlorella-PVA based bioplastic film without ultrasonic pre-treatment respectively are 5,03% and 1,32%. The heat application makes the Chlorella and PVA molecules through hydrogen bonding form more new structure thermal decomposition [7]. The same phenomenon was observed by Asada et al. (2012) where the modification of bioplastic mixture improves the compactness of the crosslinkages within the material proven by the increase of residuals left after heating. The Chlorella-PVA based bioplastic film with ultrasonic pretreatment has more residuals which shows that ultrasonication improves the bioplastic film by creating a more compact crosslinkages between Chlorella and PVA.

Surface Morphology and FTIR Analysis
Micromolecular structure of the bioplastic film was evaluated using SEM and the resulting images are shown in Fig. 3 dan Fig. 4. The images shows that ultrasonic pre-treated Chlorella resulting in more homogenized and less pores bioplastic film, creating a smoother film surface compared with the untreated Chlorella based bioplastic film. Ultrasonication also reduces the particle size of Chlorella. This indicates that ultrasonication can significantly improve the binding of Chlorella and PVA an enhance the dense homogenity of the film. FTIR analysis of the films resulting FTIR spectra that are shown in Fig. 5 and Fig. 6. The peak in 3650 -3200 cm -1 wavenumber shows that there are free hydroxyl group which causes the films to be soluble in water. The peak in 2200 -1800 cm -1 wavenumber shows that there are C-H bonds which are hydrogen bonds between amylose and amylopectin in Chlorella [8]. The peak in 1640 -1320 cm -1 wavenumber shows that there are amide group and indicates the protein in Chlorella that was not fully bonded with PVA [9]. The peak in 1320 -1140 cm -1 wavenumber shows that there are ether group (C-O) and acetal ring (C-O-C) which are the bond of PVA and Chlorella [9].

Conclusions
In this study, the ultrasonication pre-treatment of Chlorella to produce films with PVA mixture enhances perfomance of the film and could suggest its potential as a eco-sustainable food packaging plastic material. Mechanical test shows the increasing of bioplastic tensile strength up to 15,3 kgf/cm 2 and elongation percentage up to 99,63%. Field emission scanning electron microscopy test shows the increasing of bioplastic homogenity and smoother surface with less pores. Fourier transform infrared analysis shows that there are crosslinkages between Chlorella and PVA.
Thermal analysis by thermogravimetric analysis shows ultrasonication creates a more compact linkages.