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Project

Ultrasound Application for Enhanced Viscous Streams Processing and Polyurethanes Production

Processing of high viscosity streams in general, including polymer synthesis and handling in particular, represents a pivotal and rather challenging aspect of the chemical industry. In this context, the production and handling of plastics is a fundamental industry for the European Union, with millions of people being employed in this sector. Strategies are being implemented towards more sustainable materials and energy resources for the production of polymers/plastics. At the same time, new processing methods are being developed for the synthesis and post-processing of these products. Among the various ideas investigated, transition from batch to continuous polymers processing using reactive extrusion stands out. This continuous process can eventually be coupled with alternative energy sources, such as ultrasound (US), for controlled reaction activation. An ultrasound assisted reactive extrusion process could offer great improvements to the current processing demands. For instance, benefits associated with such technology include shorter processing times and therefore lower equipment size requirements, enhanced process control, improved heat transfer and mixing of the reactants. However, crucial challenges have to be dealt with before the transition to such advanced polymerization production techniques. A first obstacle, is the complexity associated with the chemistry of polymers synthesis which is in many cases not investigated in depth. Additionally, various factors can have a major impact on the reactants purity and as a result on the reaction efficiency. Moreover, application of sonication in high viscosity environments is a relatively underexplored subject.

The current thesis directly addresses these challenges, aiming in gathering insights in chemistry related aspects and important design parameters for both conventional and ultrasound-assisted polymers production. Polyurethanes (PUs) production is chosen as a representative case, due to their significant role in the polymers market. The work carried in this project falls within the broader scope of an EU funded project, called “SIMPLIFY”. While batch processing was exclusively employed herein, and reactive extrusion application was not part of the thesis scope, it is anticipated that the insights discussed here will contribute to development of this kind of state-of-the-art processes.

In the synthesis of polyurethanes, moisture control of the reactants is a necessity in order to avoid side reactions and formation of urea instead of urethane chemical bonds. Therefore, a complete investigation was performed to assess the importance of the reactants quality on the polymers produced. In this study, high molecular weight (MW) polyols were used are reactants, which were end-capped with a small molecule termed as “chain stopper”. Additionally, numerous reaction parameters were studied for this system including mixing speed, reaction temperature and catalyst concentration. Implementation of such a thorough parametrical study yielded a lot of information on reaction rate, the molecular weight distribution (MWD) and processing limitations. Furthermore, the molecular weight of the polyols employed and the choice of the stoichiometric ratio of the reactants play an important role on the properties of the final PUs produced. In order to get a better understanding of these factors, a system based on low MW polyols, without the chain terminating step was examined. Numerous polyols were selected to react with a specific aliphatic isocyanate, and the obtained PUs were analyzed with various methods. Information related to the polymers MWD and polymeric chain architecture was obtained, along with data pertaining to the thermal stability of the PUs. In addition, insights were provided into the isomers composition of the isocyanate, and emphasis was given in their contribution to the final properties of the produced polymers.

The utilization of ultrasound assistance in polymer synthesis has been reported in various cases within the literature. In particular, ultrasound energy is able to induce radical species formation that can initiate a polymerization reaction. The effects of ultrasound can also be physical and mechanical, and are associated with improved mixing of the reactants and fast heating of the solution. Nevertheless, the polyurethane production based on ultrasound assistance has not been extensively studied.

Prior to application of US in a reactive mixture, non-reactive solutions were investigated to obtain better understanding of the influence of high viscosity on the US energy field distribution. The methodology applied included both experimental and numerical work. The experimental findings revealed the impact of the solutions properties, and especially the viscosity and the surface tension, on the cavitationally active zones location and intensity. Important design parameters such as the rated power of the US device, the positioning and diameter of the US transducer, and the reactor capacity were studied and the information obtained provides useful guidelines that should be taken into consideration during the design phase of US-assisted systems for the processing of increased viscosity streams. It was shown that the energy conveyed by US exhibits a very dynamic nature, therefore, the thorough mapping of the chosen experimental set-up is a crucial step. Additionally, numerical tools were utilized to theoretically predict the pressure distribution within a representative system. Although, the development of modelling tools keeps constantly evolving, the approach taken here shown a reasonable level of compatibility with experimental data. Having numerical tools capable to predict the impact of important design aspects has great potential in easing up process design steps and avoiding time consuming experiments.

Finally, a proof-of-concept test was conducted to assess the feasibility of employing ultrasound for the synthesis of PUs on a small scale, using low-viscosity polyols. The polymers produced using the US-assisted technique were found very similar to the polymers synthesized with the traditional “silent” process, in terms of MWD and polymer chain backbone. US benefits included improved mixing of the reactants and fast heating of the solution. The silent and US processes were tuned to result in similar heating profiles, and it was shown that US energy demands are considerably lower. This study can therefore serve as a concrete basis for future research endeavors and investigation of various reactants combinations.

Date:12 Jun 2019 →  5 Dec 2023
Keywords:ultrasound, reactive extrusion, process intensification, polymerisation, polyurethane
Disciplines:Intensification
Project type:PhD project