Thermal stability investigation of recycled polyethylene terephthalate (PET) through dynamic rheological measurements

Abstract

Today, the use of polymers is becoming more and more common in the industry. Petroleum-derived polymers are frequently used in the industry because of their low cost, easy processing, availability, and easy application. With the growth of the plastics industry, some environmental problems have arisen. One of the biggest problems is that plastics take hundreds of years to disappear completely from nature as an example, a PET bottle wastes disappears completely from nature within 400 years. Due to all these ecological concerns, the industry has focused on recycling of petroleum-based polymer wastes. Recycling of plastic waste is an easy process, and it reduces the amount of plastic waste by re-adapting the plastic waste accumulated in nature to the industry. Today, 30% of the plastics produced can be recycled. However, the recycling of 60% of the produced plastics is what has been targeted for 2030. Therefore, interest and investments increase day by day recycling. The recycling process offers a promising solution to ecological problems caused by the excessive use of petroleum-based polymers that do not degrade in nature. Poly (ethylene terephthalate) is a semi-aromatic thermoplastic polyester produced by the poly esterification reaction of ethylene glycol and phthalic acid / anhydride. PET is one of the mostly used polymers in the industry due to its high chemical, mechanical and thermal resistance properties. PET is widely used in the production of bottles and food packaging. As a result of these applications, tons of PET waste are generated every year. In addition, PET is not suitable for some applications due to its slow crystallization rate, but studies have increased the crystallization rate of recycled PET. This shows that the field of application can expand. Recycling processes have advantages as well as various disadvantages. However, these disadvantages have been eliminated with the adoption of newly developed methods. When the properties of the recycled PET are compared with those of PET, some features appear to be decreased. When a material is recycled, the biggest loss will occur at the molecular weight. This results in the molecular weight of recycled materials being lower than the molecular weight of the raw materials. The loss of molecular weight in the structure of PET affects many other properties simultaneously. In particular, the viscosity of PET decreases with the loss of molecular weight. If the viscosity drop is too high, this will seriously affect the processability of PET. This is because of the fact that when the material is exposed to any thermal temperature, it will flow rapidly and accordingly will be difficult to shape. In addition, it will be affected by mechanical properties. The mechanical properties of recycled PET are low compared to raw PET. Many methods are presented in the literature in order to solve all these disadvantages and improve the properties of recycled PET. They can be modified with chain extenders when processing recycled polymers by extrusion or other methods in order to be used in the industry and to improve their properties. With the use of chain extenders in recycled polymers, the molecular weight is increased quickly and effectively. Even a small use of the chain extender causes huge effects. In addition, this method is fast, economical and easy to apply. Another method used to improve the properties of recycled polymers is the preparation of polymer blends. A polymer blend is a combination of two or more polymers that are physically mixed to obtain a single phase. This means that instead of obtaining the properties of each polymer individually, a set of properties is obtained by blending several polymers. Therefore, not all polymers may show their desired properties. By using another polymer with better properties and mixing with the other polymer, new polymeric materials having purposeful properties can be obtained. As a result of the studies, it has been clearly observed that the proper polymer blends and the proper chain extenders have positive effects on the mechanical properties, thermal properties and melt rheological properties of the polymers by the use of new polymers. In this study, our aim is to analyze and improve the thermal stability of recycled PETs after applying the recycling process. It is to show that recycled materials can be used as raw materials after their properties have been improved and become available for industry. Recycled polymer loses its properties during the recycling process, and when an extra thermal process is applied, many problems occur in the industry. Polymers are exposed to temperature by many different methods in order to be processed and shaped as final products. In addition, there are many parameters that affect on thermal behavior,when exposed to temperature. One of these parameters is the shear rate or frequency and in this way there are many process parameters. Therefore, the melt thermal stability of the materials is very important in terms of end product properties and processability. Dynamic rheology measurements can be made to determine these properties. In this context, in order to determine the processability of recycled PETs, their dynamic rheological properties were examined. Thus, how recycled PET will behave in melt state is examined. In these trials, how many parameters will affect the melt behavior of recycled PET is examined. First, the linear viscoelastic region of recycled PET was determined as 1% by strain sweep tests. This test is performed to identify the region where the viscoelastic properties of the material do not change when the strains applied. Time sweep tests were performed at different temperatures to identify thermal stability. In this way, it is possible to observe that how many percentage degredation occured for material. During the time sweep tests, different strains were also used as parameters. Frequency sweep tests were performed to determine the melt rheological behavior of recycled PET whose thermal stability was determined. In this way, rheological behavior, which is of great importance in terms of machinability, has been determined. Finally, melt behaviors of recycled PET were determined by applying stress growth test. After this point, the disadvantages that occurred with the recycling process were also observed in the structure of recycled PET. The dynamic complex viscosity is quite low. As a result of the studies, it has been observed that both the preparation of polymer blends and chain extenders have positive effects on the mechanical properties, thermal properties and rheological properties of polymers. Our aim in this study is to show that the melt rheological properties of recycled PET are improved and available for industry. In this context, we examined the dynamic rheological properties of recycled PETs to determine their processability. Thus, we have observed how rheological properties change or can be changed as a result of the recycling process. Disadvantages caused by the recycling process were also observed in the structure of recycling PET. Most importantly, the decrease in viscosity caused by loss of molecular weight was observed for recycled PETs during time sweep rheological experiments indicating its low thermal stability that could negatively affect the melt processing. After this stage, the most important step was to improve the thermal stability with the use of chain extenders and blending. For this purpose, 0.4 wt% and 0.8 wt% of chain extenders were melt mixed with recycled PET using a twin screw extruder and the thermal stability and these compounds were monitored. Considering these results, it has been observed that with the addition of a chain extender, the rheological properties change and the processability properties of the recycled PET vary as well. Moreover, blends of recycled PET with polybutylene terephthalate (PBT) at the blending ratios of 25-75%, 50-50%, 75-25% were prepared through melt blending in a twin-screw extruder. This was because PBT is a polyester polymer that has very similar to the structure of PET. Furthermore, it has very good mechanical, chemical and machinability properties that could also improve the processability and crystallization rate of recycled PET which are considered as PET's drawbacks. In this way, the effects of the prepared polymer blends on the rheological properties have been examined. Although not being used simultaneously, the preparation of the polymer blends and the use of a chain extender, could both contribute to the rheological properties of recycled materials, a change in their rheological behavior and an increase in their processability properties. In addition, by using different temperatures, the effect of the temperature, that is thermal stability, on the melt rheological properties of all materials was examined. Also, changes in the structure of recycled PET, PBT and their blends during rheological measurements were tested with the FTIR device. It has been concluded that polyesterification reacts to temperature and the use of a chain extender and therefore an increase in viscosity was observed.