Studies on electrospun poly (lactic acid) fibers and low density poly (ethylene) / thermoplastic starch blends as food packaging materials

• Main Author: Natarajan, Lakshmi
• Other Authors: Aravind Dasari
• Format: Theses and Dissertations
• Language: English
• Published: 2018
• Subjects: DRNTU::Engineering::Manufacturing::Polymers and plastics
• Online Access: http://hdl.handle.net/10356/74018
• Institution: Nanyang Technological University

Polymer based materials have been widely used in food packaging applications due to their flexibility in processing, good sealing properties and transparency. However, most of the polymer based food packaging materials are poor in their barrier properties which could hinder their effectiveness in maintaining the quality of packed foods. Employing a bio-based functional layer/coating on a polymeric layer as food packaging material could be a potential way to address this challenge. As compared to traditional cast films, electrospun polylactic acid (PLA) fibers is a potential bio-based candidate to be used as a functional layer/coating because these fibers have high surface to volume ratios and the surface properties can also be altered easily by the addition of nanomaterials during the electrospinning process. However, mechanisms of obtaining electrospun PLA fibers with different surface morphologies (to tune the desired functionality) and the role of these surface characteristics on moisture sorption and microbial growth (moisture and microbes present in food packaging environments) have not been looked at. Hence in this thesis, the first major research focus is dedicated on elucidating the formation mechanisms of varying degree of pores on PLA fibers by changing the electrospinning solution and process parameters. The role of a combination of parameters, such as solvent vapour pressure, solvent miscibility and interaction with water, solubility and relative humidity, in governing pore formation is demonstrated. The results indicated that conventional mechanisms reported earlier such as thermally induced phase separation and vapour induced phase separation were not responsible in generating pores. Instead it was concluded that solvent miscibility and interactions with water caused by high relative humidity environment (moisture, a non-solvent of the polymer) were relatively more important than just solvent volatility. The content of highly volatile solvent in the solution determined the nature of pores (spherical/ elliptical) because this highly saturates the jet-air interface during electrospinning. Moreover, fibers with and without surface pores and with nanoparticles like Sepiolite (known to absorb moisture easily) and ZnO (known to have antimicrobial activity) are employed in order to understand how the porosity and hydrophobicity of fibers affect moisture sorption and microbial activity. Electrospun fibers were found to be hydrophobic and moisture sorption results indicated very low amounts of moisture uptake despite the increased Lakshmi Natarajan surface area of the fibers caused by porosity. But the sorption levels were 3-5 times higher than the cast film counterparts. Microbial test results indicated that hydrophobicity played an important role in initial bacterial adhesion to the substrate. This was demonstrated by using fibers with different levels of hydrophobicity and compared with a hydrophilic surface obtained by spraying of sepiolite particles on the surface of fibers. All fibers except those with ZnO were observed to be bacteriostatic. These fibers do not aid in moisture scavenging but they do not promote bacterial growth. Electrospun fibers cannot be stand-alone packaging materials and they have to be used along with polymeric materials used in food packaging applications. Low Density Polyethylene (LDPE) is a very abundantly used food packaging material which is obtained from non-renewable resources and it takes decades to be decomposed. Addition of thermoplastic starch (TPS) to LDPE in the weight ratio of 50-50 is a potential way to generate biodegradable material which will reduce the environmental concern. However, the properties of LDPE/TPS blends are not as good as LDPE and this depends on several factors such as plasticization of starch, compatibility between LDPE and plasticized starch, morphology of the blends (phases of TPS in LDPE matrix) and processing conditions. Viscosity of blend components decides the morphology generated in polymer blends and this aspect is not well studied for LDPE/TPS blends. Therefore, the second major research focus of this thesis is to investigate the morphology, mechanical and barrier properties of LDPE/TPS blends when different grades of LDPE that differ in melt flow rate are used to prepare LDPE/TPS blends, with and without organically modified nanoclay. The results show that the use of LDPE with higher melt flow rate produces dispersed droplets of TPS in the LDPE matrix while LDPE with low melt flow rate showed no distinct phase morphologies. The presence of clay has shown improvement in the water vapour transmission rates of the blends. In conclusion, the surface properties and morphology of the potential food packaging materials, i.e. electrospun fibers as well as LDPE/TPS blends are greatly affected by the synthesis parameters.