Valorization of polystyrene wastes using natural terpenes and high pressure CO2.
- Autores: Cristina Gutiérrez Muñoz
- Directores de la Tesis: Juan Francisco Rodríguez Romero (dir. tes.), María Teresa García González (dir. tes.)
- Lectura: En la Universidad de Castilla-La Mancha ( España ) en 2014
- Idioma: inglés
- Tribunal Calificador de la Tesis: Kolio Troev (presid.), José María Escola Saéz (secret.), Ramón Bacardit Cabado (voc.)
- Materias:
- Enlaces
- Tesis en acceso abierto en: RUIdeRA
- Resumen
Larger and increasing volumes of plastics are produced yearly due to their low price, versatility and suitability for a large number of applications and uses. Unfortunately, after its use, polymers become a valuable feedstock and they must be recovered in order to prevent the environmental pollution and to preserve natural sources following the mandates of the European policies about wastes management. In fact, during the last decade, some positive trends concerning the recovery and recycling of polymers are occurring. The recycling of plastic wastes is justified enough on environmentally basis, but also is demonstrated that presents a large potential from an economical point reducing the pressure on the original sources of raw materials. The use of plastic wastes as source of high-added value products impulse the recycling alternative being a great environmental opportunity by several reasons: the reduction of the pressure on the exploitation of raw materials, resources and energy consumption, the increase of free volume in landfills, and obviously, the associated benefits (employment and lower prices). This research work is focused on the recycling of Polystyrene (PS) wastes by an environmentally friendly technology for the production of high-added value products. The mentioned polymer constituted and important issue since it is not biodegradable, massive amounts of energy and resource are consumed during its production, is very versatile. As it is used in daily life application with very low lifespan, it makes that frequently finish in landfills or in small pieces in seas and oceans. In order to promote the recycling of Polystyrene wastes, new processes should be designed to reduce the environmental impacts and costs and to improve the quality of the recovered products regarding to the traditional recycling techniques (mechanical recycling, chemical recycling or energy recovery). Thus, a technology according to the Green Chemistry principles is proposed. The new recycling process for Polystyrene wastes consists on the dissolution of the polymer in natural terpene solvents to reduce its volume (decreasing transport costs), remove contaminants, dissolve specifically the polymer from mixed plastic waste streams, avoiding its degradation. Next, the separation of the terpenes from the solutions is performed by the addition of high pressure CO2 as antisolvent which makes that the polymer precipitates while the solvent is removed. Carbon dioxide (CO2) has been the choice antisolvent because it is able to solubilise terpenes easily and thank to its sorption in the polymer, the processing is enhanced. Furthermore, CO2 has resulted to be a very promising blowing agent which encourages its use for the production of microcellular foams, a high-added value product. Moreover, it should be highlighted that any waste stream from the process is obtained because the terpenes and the CO2 are easily recovered and reused in several cycles of the process, minimizing the environmental impact. As the objective is to create new products with higher added value than the original raw material using a residue, is necessary to know the interaction between Polystyrene, terpene solvents and high pressure CO2 to anticipate the conditions in which the removal of the solvent has to be done to produce improved new materials. The research started up with the definition of the binary systems which makes the different steps of the process. Thus, in order to determine the maximum amount of treated wastes with a certain amount of solvent, the influence of the temperature, the molecular weight and the source of the Polystyrene wastes (Expanded or Extruded) on the solubility in terpenes was studied. It was concluded that terpenes are suitable solvents for the dissolution of the polymer at room temperature independently of its molecular weight and processing. On the other hand, the solubility of terpenes in CO2 is crucial since the success of the separation process depends on it. It was observed that terpenes are fully soluble in CO2 at room temperature and mild pressure. Finally, although Polystyrene is insoluble in CO2, the gas is greatly absorbed into the polymer causing its swelling and plasticization. This phenomenon entails a modification of the Polystyrene properties (viscosity, interfacial tension and glass transition temperature), which affects the processing of the polymer at high pressure. Once the binary systems had been defined and studied, it is important to know the behaviour of the ternary mixtures involved in the process (CO2/terpenes/Polystyrene) since the knowledge of the binary systems is not enough to select the operating conditions successfully. According to the binary system, the solvent removal is possible thanks to supercritical CO2 since it provides high solubility of terpenes and fully PS insolubility at moderated pressures and temperature. The influence of pressure, temperature and concentration on the solubility of the selected terpenes (Limonene and p-Cymene) in CO2 and on the sorption of CO2 into the solutions (Polystyrene/Limonene and Polystyrene/p-Cymene) were determined. From these results, the most suitable conditions for the precipitation of Polystyrene using CO2 as antisolvent were those where the antisolvent density was higher (high pressure and low temperature) and the concentration of Polystyrene in the solutions moderated. Next, the properties of the mixtures were determined since the knowledge of the phases equilibrium is too poor to define the global process. The plasticizing effect of CO2 on the polymer is also observed in terpene solutions and consequently, a deep understanding of how the properties of materials and the processing parameters affect the behaviour of the mixtures is required. The study of the viscosity, interfacial tension and glass transition temperature of the mixtures was carried out. Although the viscosity and the interfacial tension of the ternary systems were significantly affected by the antisolvent, the glass transition temperature showed similar results to those obtained for the binary system Polystyrene/CO2. Once the system was characterized and the equilibrium and properties of the ternary mixtures determined, the recycling process was studied. Two main alternatives were explored: continuous or discontinuous regime. In continuous regime, named Supercritical Antisolvent (SAS) the polymer solution is sprayed to a vessel previously filled with CO2. This process allows the control over the particle size. Nevertheless, Polystyrene is precipitated according to a wide variety of morphologies and the knowledge of the hydrodynamic of the system was crucial to understand the process. Thus, the way in which the polymer solution is contacted with CO2 affects the diffusion and mixing of the fluids and consequently the way on which the polymer precipitates. Due to the technical difficulties found along the development of the SAS process, the discontinuous one was proposed as alternative, which is focused on the foaming of the solution for the production of Polystyrene microcellular foams. In this case, CO2 behaves not only as solvent of the terpene and antisolvent of the Polystyrene, but also as blowing agent of the polymer, which allows tuning the cells size, cells size distribution and cells density of the foamed Polystyrene. The presence of the terpene promotes the mass transfer that is mainly governed by the diffusion of the CO2 into the polymeric rich phase due to the relaxation of the polymer chains. The recycled Polystyrene, obtained following this methodology, presents microcellular structure, non remarkable traces of terpene and any evidence of degradation. Finally, the economic analysis of the process was carried out to determine the feasibility of the recycling process. The traditional high costs involved in the recycling of PS wastes are balanced by the high added value of the recycled polymer, but also due to the mild operating conditions required to carry out the process. The selective dissolution technique eliminates the need to sort previously the polymer from mixed plastic waste streams. The economical benefits, together with the environmental advantages show the interest of the process for the treatment of the large amount of Polystyrene wastes produced daily.
