Obtaining dioctyl terephthalate from waste PET and its use in concrete mortar

In this study, it was aimed to obtain useful products from waste polyethylene terephthalate (PET) materials through chemical recycling and the usability of the obtained products in cement composites was investigated. In the first part of the study, dioctyl terephthalate (DOTP) was synthesized by chemical recycling of waste PET with isooctyl alcohol in the presence of zinc acetate dihydrate catalyst at atmospheric and higher pressures. The efficiency of the experimental parameters, the relations of the parameters with each other and the optimum parameter levels were determined using the refractive index, pH, viscosity, "remaining PET amount" data obtained from the reaction products by using the analysis of variance, hypothesis testing and experimental design for each output. According to the results of the statistical analysis: it was seen that the temperature and reaction time were effective on the remaining amount of PET, and it was seen that the parameters increased the effects of each other. It was observed that the amount of remaining PET decreased with the increase of temperature and reaction time, and optimum parameter levels minimizing the amount of remaining PET were found as 250°C for temperature and 15 minutes for reaction time. For comparison purposes, waste PET was reacted with distilled water to obtain terephthalic acid (TPA). The optimum parameter levels determined for this reaction were found to be 250°C for temperature and 15 minutes for reaction time. The products obtained after synthesis were characterized by a fouirer transform infrared spectrophotometer (FTIR) and nuclear magnetic resonance device (NMR). FTIR and NMR data of the products were analyzed with hierarchical clustering analysis and principal component analysis, which are multi-response statistical analysis methods, and the relationships between the samples were examined. As a result of the hierarchical clustering analysis, it was seen that the samples synthesized at 200°C clustered in a different group than the samples synthesized at 225°C and 250°C. In principal component analysis, 99.36% of the total variance could be explained with the first two principal components. It has been determined that the most effective functional groups on principal components are groups containing oxygen. By comparing the score values calculated using the basic components, it was found that the samples synthesized at 200°C had different scores than the samples synthesized at 225°C and 250°C and could be easily separated using these scores. Cluster analysis was performed on the synthesized terephthalic acid (TPA) samples for comparison purposes. As a result of the analysis, the samples synthesized at 200°C were clustered in the same cluster, and in the principal component analysis, only 76% of the total variance could be explained with the first two components, and no distinction could be made between the samples based on the principal components and temperature. In the second part, cement composites were manufactured with DOTP obtained from waste PET. These manufactured composites were soaked and cured in sulfuric acid, hydrochloric acid, nitric acid, and sodium sulfate solutions. The property changes detected after curing are as follows: 37%-70% decrease in strength in sulfuric acid solution, 34%-60% decrease in electrical resistance, up to 21% increase in UPV (ultrasonic pulse velocity), up to 35% in thermal conductivity. Up to 64% increase in specific heat and a 6–9% decrease in weight were determined. There was 52%-85% decrease in strength, 38%-70% decrease in electrical resistance, 11% increase in UPV, 21% increase in thermal conductivity, 47% increase in specific heat, and 10-13% decrease in weight in hydrochloric acid solution.In nitric acid solution, there was 38%–70% decrease in strength, 34%–60% decrease in electrical resistance, 6% increase in UPV, 36% increase in thermal conductivity, 64% increase in specific heat, and 6-9% decrease in weight.There was 22% increase in strength, 21%–46% decrease in electrical resistance, 10% increase in ultrasonic sound speed, 36% increase in thermal conductivity, 63% increase in specific heat, and 4% increase in weight in sodium sulfate solution. For comparison purposes, composites were prepared by adding terephthalic acid (TPA) to cement mortar. There was an up to 81% increase in strength, 56%–63% decrease in electrical resistance, and 10% increase in UPV in these composites.