Vidales Herrera, Jessica Vidales
(2023)
Microwave-assisted polymer synthesis of polycondensation materials for high performance coatings.
PhD thesis, University of Nottingham.
Abstract
Polymers have become important industrial materials due to their versatility and wide range of properties and applications. In 2020 worldwide plastics production reached 367 million tons. One of the challenges that this sector faces is the long reactor cycle times associated with the current manufacturing technology, which acts as a bottleneck and limits production volumes. Efforts towards process intensification based on the development of innovative systems and techniques capable to deliver faster production rates with less energy and waste are of great interest for manufactures as even small reductions in cycle time could result in significant economic and environmental benefits.
The application of microwave heating methods has become an interesting technology for the polymer industry due to the reported increased heating and polymerization rates as well as the higher final product conversion obtained compared to conventionally heated reactions at small laboratory scale. However industry lacks familiarity with this technique given the limited commercially available reactors for modest scale-up (5 L) to produce industrially meaningful samples and limited knowledge regarding microwave heating equipment design for the development of this technique at industrial scale. Potential users consider the equipment to be complex and the needed feasibility studies, equipment modification and custom design expensive.
The aim of this thesis was to deepen and broaden the knowledge of microwave-assisted polymerization reactions and study their scalability in order to contribute to their development and implementation in industry.
Owing to the importance of polycondensation reactions in polymer manufacturing and the interest of the coatings sector over alkyd resins, contributing 70% to the conventional binders used in surface coatings, an alkyd resin of commercial use was chosen as target material to conduct this project. Polycondensation reactions are characterized by the long reaction times and high temperatures needed to remove condensation-by products and reach high conversion and reaction rates, microwave heating is a technique with great potential for their optimization. This project explores the scalability of microwave-assisted polycondensation reactions from gram-scale (50 g) to kilo-scale (4 Kg) using a bespoke in-house-built hybrid reactor capable of using both heating methods and perform batch and continuous-flow syntheses.
The dielectric properties of the raw materials used for the alkyd preparation were analysed over the reaction temperature range using the Cavity Perturbation Technique (2.47 GHz) to understand their interactions with the electromagnetic field. Monomers are low microwave absorbers at room temperature and become medium heaters at reaction temperature after the melting and incorporation of solid monomers into the liquid phase, which shows that there is significant potential for volumetric heating at large scales without penetration depth limitations.
Common reported advantages found for microwave-heated polymerizations were evaluated via a comparative study between heating methods and at different scales (50 g and 4 kg – batch processing) using the alkyd resin as a model system. Particular attention was given to the similarity of experimental setups in terms of volume/mass and design as well as to the reaction temperature monitoring and control. The alkyd polymer was successfully synthetized under microwave-heating at both studied scales, this being the first time that an alkyd with commercial use has been synthetized using microwave heating particularly at kilo-scale.
Whilst no acceleration was perceived for 50 g microwave-assisted reactions, kilo-scale microwave reactions underwent a 35% reduction of reaction time (7:50 hours vs. 12:00 hours), which was measured via acid value titrations. Kilo-scale’s shorter reaction time was attributed to the effective water removal achieved by the selective heating of water molecules, which is present throughout the reaction effectively evaporating and eliminating it from the system. It was estimated that 42% of the reduction in reaction time is due to an effective water removal at the start of the condensation, which in turn leads to faster 200-230 °C heating ramp. This initial effective water removal effect was hindered in small-scale reactions due to mass transfer limitations associated with the small quantities of water (produced) and xylene (used) involved, in addition to the need of pre-warming the experimental setup glassware via xylene/water reflux before the reflux is set.
The polymer growth analysis performed in GPC showed different formation patterns between heating methods, microwave experiments presenting more activity at the start and mid-final stage of the reaction. This was attributed to the selective heating of oligomer species at the start of the reaction and the favoured formation of short polymer chains over long ones in the middle stage of the reaction, due to the reduced dielectric properties of larger molecular weight species. Consequently microwave reactions present lower high molecular weight chains and viscosity in the late stage of the synthesis, which facilitates the reaction between polymer chain ends and leads to the formation of larger polymers than conventional experiments. Microwave heating reached higher molecular weights at both reaction scales for equivalent AV. The difference in molecular weight is more significant at small-scale (Mw: 29.7kDa vs. Mw: 18.0kDa) than kilo-scale (Mw: 33.8kDa vs. Mw: 30.3kDa). The different polymer growth between heating methods has been described for the first time in this thesis, providing a possible explanation to the higher molecular weights reached in microwave syntheses.
The use of continuous-flow processing was studied as a microwave-heating additional technique for retrofit applications, which could facilitate the introduction of microwave technology in industry with fewer modifications to existing current reactors in the future. Reaction time of conventional reactions was the same for both processing modes, while microwave processing resulted in a 22% reduction, saving 2:35 hours in a conventional process.
The difference in reaction time between batch and continuous flow microwave synthesis is associated with the absence of initial effective water removal in the continuous-flow scenario. This was explained by the lower relative intensity used compared to batch experiments (0.45 W/g vs. 0.22-0.32 W/g) leading to slower heating rates and the distance between the microwave applicator and the distillation column in this configuration which hindered the fast elimination of water vapour by re-condensing it through the circulation pipes. The presented hybrid heating reactions establish the bases for further process optimization. This being the first time that an alkyd with commercial use has been synthetized using microwave heating at a kilo-scale and both processing methods. These are promising results demonstrating that polycondensation reactions could benefit from even partially microwave-assisted processes.
A preliminary approach to progress monitoring using in-line dielectric measurement techniques was given by off-line CPT measurements of the dielectric properties of the alkyd resin, which displayed a decreasing trend. The obtained values provided information of the expected measurement rage (ε''=0.45 – 0.21 and Tanδ=0.12 – 0.06 at 2.47 GHz). It is noticed that at this frequency range the variation of the dielectric properties is small, particularly in the middle to late stages of the reaction the relationship of dielectric loss/conversion/acid value accuracy can be difficult to achieve. Other measuring frequencies should be considered for the purpose or reaction monitoring, reducing the contribution of water presence in the system and providing a wider spread of values.
Microwave heating was investigated as a potential technique for polymer manufacturing contributing to their process intensification by enhancing energy transfer, increasing heating rates and reducing process time, tackling long reactor cycle times and enhancing production rates.
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