Effect of Zeolites on the Structural and Thermal Properties of PLA/PEG Blends

Turkchem 27 Aug 2018 61 6 dk okuma

TURKCHEM

Abstract

This study investigated the effect of synthetic zeolite added to composites prepared from polylactic acid (PLA) and polyethylene glycol (PEG) components to improve their structural and thermal properties. Zeolites were added to the polymer blends at ratios of 5, 10 and 20%. FT-IR (Fourier Transform Infrared Spectroscopy), SEM (scanning electron microscopy), TGA (thermogravimetric analysis), DSC (differential scanning calorimetry) and water holding capacity analyses were performed to determine the characteristic properties of the resulting polymer composites. Analysis results showed that zeolites added to PLA/PEG composites improved thermal stability and increased water holding capacity.

1. Introduction

The term polymer is derived from the Greek words "poly" meaning many and "meros" meaning unit or part. A polymer is a large macromolecule formed by the repetition of small units called monomers. Due to many properties possessed by polymers (high strength, ease of processing, low cost, etc.), they are currently used in many fields such as automotive, household appliances, medical materials and packaging. Non-recyclable polymer materials used in these fields create an increasingly serious waste problem. Especially plastic materials widely used in the packaging sector are mostly petroleum-based and cause significant environmental damage. These petroleum-based plastics such as polyethylene (PE), polystyrene (PS) and polyethylene terephthalate (PET) do not decompose in nature for many years, leading to increased waste and CO2 levels in the atmosphere. Biodegradable polymers can decompose in suitable composting environments in a short time compared to petroleum-based polymer materials. Since petroleum-based polymer materials, which are the primary cause of waste generation, take years to decompose in nature, it is believed that this problem can be largely solved by replacing traditional polymers with biodegradable polymer materials[1]. Among biodegradable polymers, polylactic acid (PLA) has mechanical and physical properties that are largely comparable to petroleum-based polymers such as polystyrene, polyethylene terephthalate and polypropylene. Since PLA has properties close to traditional polymers used in the packaging sector, it appears that PLA can be used in this field[2]. Although PLA has various advantages such as being biodegradable, environmentally friendly, biocompatible, easily processable, transparent, high modulus and strength, it cannot be used to the desired extent in many applications today due to its low impact resistance, poor gas barrier properties, low thermal resistance and brittleness. In recent years, to improve PLA properties and reduce cost, various polymers and additives have been used through a composite material approach to develop PLA applications. So far, polymer blends and composites have been produced using various biodegradable polymers such as starch, protein and polycaprolactone and natural additives such as wood, chitosan and flax. For this purpose, many micro and nano-structured additives such as clay, silica and montmorillonite have also been used to improve PLA properties[3]. 2. Material and Method 2.1. Material The materials used in the study, the suppliers and their properties are presented in Table 2.1.

Table 2.1. Materials used in the study and their properties

2.2. Method

In this study, composites were prepared as films using the solvent casting method. Initially, PLA and zeolite were dried at 60°C for 24 hours to remove moisture. A solution containing 5% PLA was prepared with chloroform, and PEG was added to contain 20%. Zeolite 13X was added to the obtained PLA/PEG mixture at ratios of 5, 10 and 20%. The solutions were stirred on a magnetic stirrer and in an ultrasonic bath to obtain a homogeneous structure. The polymer mixtures poured into petri dishes were dried by keeping them at 40°C for 3 days and prepared for analysis.

3. Results and Discussion

This section presents the characterization results of composite films prepared with PLA/PEG and zeolite. Figure 3.1 shows the FT-IR spectra of neat PLA, PLA/PEG, PLA/PEG containing 5% zeolite, PLA/PEG containing 10% zeolite and PLA/PEG composite films containing 20% zeolite. Peaks observed at 3330 cm⁻¹ and 1637 cm⁻¹ wavelengths originate from the stretching and deformation vibrations of the hydroxyl (-OH) group in zeolite respectively, and the intense peak observed at 947 cm⁻¹ shows the asymmetric stretching of Si-O and Al-O bonds belonging to SiO₄ and AlO₄ tetrahedra. The intensity of these peaks increased with zeolite addition[3].

Figure 3.1. FT-IR spectra of PLA films

Figure 3.2 shows SEM images to determine the morphological structure of PLA films and zeolite. According to analysis results, In Figure 3.2.A, zeolite has a cubic shape, in Figure 3.2.B, PLA has a smooth and uniform surface, in Figure 3.2.C, PLA/PEG films have an appropriate and homogeneous surface capable of forming a blend, and in Figure 3.2.D, zeolite-containing films showed very good dispersion[4].

Figure 3.2. SEM images of zeolite and PLA and PLA/PEG films. A) Zeolite B) PLA C) PLA/PEG D) PLA/PEG/Z10

Table 3.1 shows the DSC results of the polymer films prepared. According to analysis results, the Tm value did not change with zeolite addition. The Tc value decreased from 123.56°C to 75.33°C due to 20% PEG effect. The addition of PEG to PLA, which has a low crystallization rate, increased this rate. Additionally, as the zeolite content in the polymer films increased, crystallization temperatures increased[5].

Table 1. DSC Film Results

Figure 3.3 shows the TG analysis results of PLA films. With PEG addition, the chain mobility and flexibility of PLA polymer are positively improved. When evaluated according to the curves in the TG graphs, it is seen that the thermal stability of PLA decreases with PEG addition[6,7]. It was determined that the thermal stability of composites formed by adding zeolite to PLA/PEG blends increased.

Figure 3.3. TG graphs of polymer films

Figure 3.4 shows the water holding capacity of zeolite-containing PLA/PEG films at 30°C. According to test results, it was observed that the water holding capacity of PLA/PEG composite films increased with zeolite loading.

Figure 3.4. Water holding capacity of composite films

4. Conclusion

PEG is added to improve some weak properties of neat PLA. However, while PEG improves some properties of PLA, it negatively affects others. Zeolite was chosen as an additive to eliminate these negative properties. In this study, aimed at determining the characteristic properties of composite films prepared with zeolite addition and identifying the most suitable composite ratios, the composite containing 10% zeolite gave optimum results. It was observed that PEG reduces the thermal resistance of PLA, while zeolite increases it. While PEG lowered crystallization and melting temperature, zeolite addition increased these values. Looking at the water holding capacity of the films, it was determined that due to the hygroscopic nature of zeolite, the water holding capacity of the polymer films increased. Furthermore, by adding additives to the polymer matrix, it was observed that the porous structures formed between the matrix and the additive were effective in water retention in the films. Feza Geyikçi Department of Chemical Engineering Ondokuz Mayıs Üniversitesi Buğçe Özoğul Department of Chemical Engineering Ondokuz Mayıs Üniversitesi