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Modifying the Mechanical and Rheological Properties of Recycled Polylactic Acid for use in PBAT/r-PLA/TPS Biodegradable Film

Enayati, Mahdi | 2024

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 57265 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Bagheri, Reza
  7. Abstract:
  8. The environmental concerns in today's world, encourage the people involved in the plastic industry, along with the development of recycling culture, to produce and consume bio-based and biodegradable plastics. In this regard, during the last two decades, polylactic acid (PLA), which is a bio-based and biodegradable polymer, has experienced a high growth rate of production and is used in various applications such as packaging, textile and additive manufacturing. Along with the growth of the consumption of this polymer, the knowledge and development of its recycling methods have also been considered by researchers. Considering the expansion of the use of PLA blended with another biodegradable polyester, i.e. polybutylene adipate terephthalate (PBAT) as well as thermoplastic starch (TPS) in the production of compostable films, this research aims to use recycled polylactic acid (r-PLA) instead of PLA in the aforementioned ternary blend. In the first part of the current research, an attempt was made to partially compensate for the significant drop in r-PLA molecular mass in the mechanical recycling process with the help of a natural chain extender, namely epoxidized linseed oil (ELO). In the second part of the research, r-PLA modified with two polymers PBAT and TPS was blended and used in the production process of blown film. Blends were made by twin screw extrusion method. Various characterization tests including FTIR spectroscopy, melt rheology, differential scanning calorimetry, tensile and scanning electron microscopy were used to evaluate the results and analyze the data. The results of the first stage showed the positive effect of chain extender in small amounts, so that by adding epoxidized linseed oil to r-PLA up to 1 wt%, the complex viscosity increased continuously and the changes in mechanical properties were limited in this range. However, at higher values of the chain extender, a drop in complex viscosity and tensile strength was observed, while at the same time, an increase in the strain at break was observed. The observation of these results was attributed to two mutual but simultaneous effects of epoxidized linseed oil. In addition to the effect of chain extension, this oil simultaneously acts as a plasticizer and facilitates the movement of chains. Therefore, in amounts higher than 1 wt%, the effect of plasticizing prevails and the mentioned results are observed. In this way, the optimal amount of epoxidized linseed oil to modify the structure of r-PLA molecule in this stage of the research was determined to be equal to 1 wt%. In these optimal conditions, the tensile strength decreased from 42 to 40 MPa and the fracture strain increased from 4 to 8%. Of course, it should be noted that the growth of the r-PLA molecular mass with the addition of ELO in this research was partly due to the branching growth of the molecules. In the second part of the research, with the aim of comparing PLA, r-PLA and modified r-PLA, three blends containing 60 wt% of PBAT and 15 wt% of TPS were made. In order to evaluate the effect of chain extender, a fourth blend in which ELO was added during the preparation of PBAT/r-PLA/TPS blend was also made. In the following, films were made with these four blend using the blowing method. The high melt strength of the blend containing modified r-PLA caused the film with the lowest thickness (190 microns) to be made in this way. Improving the compatibility of the second phase particles with the background (PBAT) in this blend was also found to be effective in increasing the melt strength. The tensile test results showed that the modification of recycled polylactic with epoxidized linseed oil in the ternary blend increases the tensile strength from 6.5 to 9 MPa and the strain at break from 70 to 190%. The results of this research pave the way for a practical method for using r-PLA in the production of PBAT-based biodegradable film by blowing method and with acceptable properties for industrial applications
  9. Keywords:
  10. Polylactic Acid ; Recycling ; Vegetable Epoxidized Linseed Oil ; Biodegradable Film ; Thermoplastic Starch ; Rheological Properties ; Mechanical Properties

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