Production of PLA based polymeric composite foams by compression moulding and investigation of their properties

The vast majority of polymers which are produced from fossil fuels, become waste in the environment where they do not degrade after the end of their service life, and similar processes are repeated in terms of both environmental damage and consumption of limited fossil fuel resources in each new raw material production process. This vicious cycle has caused that given rise to the investigations of environmental, bio based and biodegradable polymers from renewable sources in recent years. Although biopolymers initially were used only in biomedical applications due to their biocompatibility and biodegradability, they have started to be considered as an alternative to petroleum based polymers in long-term automotive and electronic applications due to their physical and mechanical behaviors, which has been better understood in recent years. Polylactic acid (PLA) is an aliphatic polyester which has been investigated in many applications in recent years with high tensile strength, high hardness, easy processability and has comparable properties with conventional petroleum-based polymers. In addition to its advantages, PLA has a number of limiting properties such as high cost, low toughness, low thermal resistance and low crystallization rate. However, it has been found in the studies that these limitations can be solved by preparing mixtures of different polymers, reinforcing fibers, micro and/or nano fillers and selected additives, and composite materials can be prepared for the desired purpose. When the literature studies are evaluated, it has been seen that the production of foam materials which are focused on as a result of the lightness and CO2 emission limitations, which are especially important for the automotive sector, is a field that has been researched and studied for biopolymers and the studies carried out with both physical and chemical foaming agents have more important in recent years. In this thesis, it is aimed to develop lightweight, environmentally friendly, high performance PLA polymeric composite foams which can be used in automotive sector by using twin screw extruder and compression moulding methods. For this purpose, polyamide 6 (PA6) and polypropylene (PP) polymers were used together with PLA and firstly PLA/PP and PLA/PA6 polymeric composite structures were prepared at 20:80, 30:70, 40:60 and 70:30 ratios in order to determine the optimum polymer ratios. Optimum polymer ratios were detected after the evaluation of the physical, thermal, mechanical and morphological properties and the changes in these properties of these composite structures prepared by compression moulding. The optimum polymer ratio was determined as 30:70 for PLA/PP and PLA/PA6 mixtures, and it was decided to continue the studies after this stage by using the mixtures prepared with these ratios. However, in order to better understand the effects of the chemical foaming agent and the compatibilizing agent, it was decided to work with the ratio of 70:30 as well as 30:70. In the second stage of this study, in order to determine the chemical foaming agent ratio, polymeric foam structures were prepared by twin screw extruder and compression molding methods using 1%, 1.5% and 2% by weight of chemical foaming agent and the physical, thermal, mechanical and morphological properties of the samples and changes in these properties were evaluated. The optimum chemical blowing agent ratio was found to be 1.5% for PLA/PP and PLA/PA6 blends. In the last stage, after the optimum polymer ratios and optimum chemical foaming agent ratios are determined for the structures, in order to eliminate the incompatibility in foamed binary polymer systems, studies have been conducted on the use of compatibilizing agents. For this purpose foamed PLA/PP and PLA/PA6 blends were prepared using 1, 3 and 5% compatibilizing agents and the effects of the use of compatibilizing agent on physical, thermal, mechanical and morphological properties were evaluated. While the optimum ratio of compatibilizing agent was determined as 1% by weight for PLA/PP mixtures in foam structure, this ratio was determined as 3% by weight for PLA/PA6 mixtures in foam structure.