polymers Review Application of Fluids in Supercritical Conditions in the Polymer Industry Karol Tutek, Anna Masek * , Anna Kosmalska and Stefan Cichosz Institute of Polymer and Dye Technology, Faculty of Chemistry, Lodz University of Technology, Stefanowskiego 12/16, 90-924 Lodz, Poland; [email protected] (K.T.); [email protected] (A.K.); [email protected] (S.C.) * Correspondence: [email protected] Abstract: This article reviews the use of fluids under supercritical conditions in processes related to the modern and innovative polymer industry. The most important processes using supercritical fluids are: extraction, particle formation, micronization, encapsulation, impregnation, polymerization and foaming. This review article briefly describes and characterizes the individual processes, with a focus on extraction, micronization, particle formation and encapsulation. The methods mentioned focus on modifications in the scope of conducting processes in a more ecological manner and showing higher quality efficiency. Nowadays, due to the growing trend of ecological solutions in the chemical industry, we see more and more advanced technological solutions. Less toxic fluids under supercritical conditions can be used as an ecological alternative to organic solvents widely used in the polymer industry. The use of supercritical conditions to conduct these processes creates new opportunities for obtaining materials and products with specialized applications, in particular in the medical, pharmacological, cosmetic and food industries, based on substances of natural sources. The considerations contained in this article are intended to increase the awareness of the need to change Citation: Tutek, K.; Masek, A.; the existing techniques. In particular, the importance of using supercritical fluids in more industrial Kosmalska, A.; Cichosz, S. methods and for the development of already known processes, as well as creating new solutions Application of Fluids in Supercritical with their use, should be emphasized. Conditions in the Polymer Industry. Polymers 2021, 13, 729. https:// Keywords: supercritical fluid; extraction; particle formation; encapsulation; micronization; polymer- doi.org/10.3390/polym13050729 ization; polymer; foaming; impregnation Academic Editors: George Z. Papageorgiou and Andrzej Rybak 1. Introduction Received: 22 January 2021 The supercritical state of chemicals has been known for less than two centuries. In 1822, Accepted: 24 February 2021 Published: 27 February 2021 Charles de la Tour discovered a different “state” of matter besides solids, liquids and gases. Liquids above the critical value of temperature and pressure (called the critical point) exceed Publisher’s Note: MDPI stays neutral the limit beyond which their properties, such as viscosity, density, dielectric permittivity, with regard to jurisdictional claims in are the results of both of these states of aggregation. However, they do not undergo any published maps and institutional affil- processes of evaporation, condensation or sublimation; i.e., phase transitions. The only iations. known naturally occurring supercritical substance is the water found in hydrothermal vents, where there is increased temperature due to the proximity of magma and increased pressure due to the limited capacity of underground reservoirs. The liquid state is characterized by rather strong intermolecular interactions, weak enough to displace them. This is also due to an increase in the internal energy of the Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. molecules. There is a certain balance between the internal energy and intermolecular inter- This article is an open access article actions. As a result, we see slight changes in the volume of the liquid with the temperature distributed under the terms and change, as well as the possibility of taking the shape of the vessel, the characteristic viscos- conditions of the Creative Commons ity and the surface tension. On the other hand, the gaseous state is the result of an increase Attribution (CC BY) license (https:// in internal energy and its dominance over the intensity of intermolecular interactions. It is creativecommons.org/licenses/by/ also for this reason that the clear free movement of gas molecules throughout the volume of 4.0/). the vessel is possible. Among the properties, it should also be mentioned that the gas can Polymers 2021, 13, 729. https://doi.org/10.3390/polym13050729 https://www.mdpi.com/journal/polymers Polymers 2021, 13, x FOR PEER REVIEW 2 of 18 Polymers 2021, 13, 729 2 of 17 the volume of the vessel is possible. Among the properties, it should also be mentioned that the gas can take the shape of a vessel, and the gas volume also depends on its tem- take the shape of a vessel, and the gas volume also depends on its temperature (assuming perature (assuming a constant pressure). It is possible to expand and compress gases. The a constant pressure). It is possible to expand and compress gases. The important thing important thing is that temperature determines the average amount of internal energy per is that temperature determines the average amount of internal energy per molecule in a molecule in a given phase, and pressure is responsible for the probability of these mole- given phase, and pressure is responsible for the probability of these molecules colliding, cules colliding, intermolecular interactions and the distance between them. Increasing the intermolecular interactions and the distance between them. Increasing the temperature in temperature in the system makes the molecules that have more energy detach from each the system makes the molecules that have more energy detach from each other more easily, other more easily, and the increase in pressure results in a decrease in the distance be- and the increase in pressure results in a decrease in the distance between the molecules tweenand their the moremolecules frequent and collisions, their more which frequent in the collisions, next step leadswhich to in energy the next transfer step leads between to energythem. Bothtransfer these between parameters them. areBoth somewhat these parameters antagonistic are whensomewhat looking antagonistic at the effects when of lookingtheir changes at the (Figureeffects of1). their What changes the phase (Figure is at 1). a given What moment the phase depends is at a ongiven their moment value. dependsPreviously, on fortheir technical value. Previously, reasons, mainly for techni processescal reasons, in the liquid mainly phase processes and less in frequentlythe liquid phasein the and gas phaseless frequently were used. in the It happened gas phase because were used. the reactionIt happened was easybecause to carry the reaction out and wasbecause easy ofto the carry knowledge out and ofbecause the processes of the knowledge leading to it.of However,the processes the discoveryleading to of it. Charles How- ever,de la the Tour discovery and the developmentof Charles de ofla knowledgeTour and the about development the phenomena of knowledge occurring about in super- the phenomenacritical conditions occurring by successivein supercritical scientists conditions made by it possible successive to usescientists supercritical made it fluids possible not toonly use as supercritical a laboratory fluids curiosity not butonly also as asa labo practicallyratory curiosity and industrially but also important as practically processes, and industriallythanks to which important it is possible processes, to obtain thanks products to whic inh an it efficient,is possible qualitative to obtain and, products importantly, in an efficient,profitable qualitative manner. and, importantly, profitable manner. Figure 1. Schematic phase diagram of the dependence of the density changes of the medium Figure 1. Schematic phase diagram of the dependence of the density changes of the medium de- depending on the pressure and temperature. Density changes are graphically represented as changes pending on the pressure and temperature. Density changes are graphically represented as changes inin the the blue blue dot dot density. density. Thermodynamic changes cause changes in the properties of substances. The density, Thermodynamic changes cause changes in the properties of substances. The density, viscosity and diffusivity vary significantly. In the case of supercritical fluids, it is visible viscosity and diffusivity vary significantly. In the case of supercritical fluids, it is visible that the values of these parameters are intermediate between the liquid and gas phases, that the values of these parameters are intermediate between the liquid and gas phases, which can be seen in the Table1 below. which can be seen in the Table 1 below. Table 1. Comparison of physical parameters in different physical states [1,2]. Table 1. Comparison of physical parameters in different physical states [1,2]. Phase Density (kg/m3) Viscosity (µPa × s) Diffusivity (mm2/s) Phase Density (kg/m3) Viscosity (µPa × s) Diffusivity (mm2/s) GasGas 1 1 10 10 1–10 1–10 SCFSCF 100–1000 100–1000 50–100 50–100 0.01–0.1 0.01–0.1 LiquidLiquid 1000 1000 500–1000 500–1000 0.001 0.001 AnAn important important factor factor in in selecting selecting fluids fluids for for supercritical supercritical conditions conditions is is the the relatively relatively lowlow pressure pressure and and temperature temperature reached reached at at the the cr criticalitical point, point, which which makes makes the the process process cost- cost- Polymers 2021, 13, 729 3 of 17 effective.