(34d) Intensification of Polymer Production Using Ultrasound: The Polyurethane Case As Example

Introduction: A plethora of existing chemical processes has benefited from the implementation of alternative energy sources, such as ultrasound (US). Development of such novel processes is among the most promising and at the same time challenging areas of Process Intensification¹. Application of US can be found in chemicals and polymers synthesis, in crystallization and in various separation processes, among others². The effects of ultrasound on the production of various chemicals can be physicochemical or mechanical. When a sound wave propagates through a fluid, the violent implosion of numerous gas bubbles is caused, which -locally- triggers extreme temperature and pressure conditions³. These bubble collapses termed acoustic cavitation result in a series of other phenomena, such as free radical formation, acoustic streaming, high shear stress, mixing in micro-scale, boundary layer disruption and turbulence at very local, small scale^2,4,5. Focusing on US assisted polymerizations, polymers produced based on the ring opening or free radicals initiation mechanisms have been reported to benefit from the radical species generation caused by US⁴. On the other hand, very few examples exist for polymers produced based on other synthesis mechanisms, such as step growth reactions⁶. Polyurethanes (PUs) –a very important class of polymers– are produced based on the latter mechanism, and are used in a great variety of applications as the final product (elastomers, fibers, foams, etc) or incorporated in a product matrix (e.g. in paints and coatings)⁷. Application of high power US on PUs production reportedly included reaction rate improvement and potential better reactants distribution due to enhanced heating and mixing of the bulk⁶. Nevertheless, the exact mechanism behind these benefits is not yet clarified.

Objective: The goal of the current work is twofold: firstly, obtaining a better understanding of the PU chemistry by thorough investigation of the influence of reaction conditions (temperature, reactants stoichiometric ratio and reactants composition) on the final properties of the PUs produced; secondly, the in-depth investigation of the effects of applying ultrasound for the production of such polymers, compared directly to the conventional thermal processing.

Materials & methods: Various diols were used including ethylene glycol (EG), diethylene glycol (diEG), polyethylene glycol with molecular weight of 200 and 400 g/mol (PEG200 or PEG400). The diisocyanate selected was 4,4-Methylenebis(cyclohexyl isocyanate), referred to as HMDI for simplicity hereafter. HMDI was used as received (mixture of isomers) or for selected cases the trans-trans (termed t,t HMDI herein) form was precipitated, separated, purified and used for the polymerizations. In some reactions bismuth carboxylate was added as the catalyst.

Bulk or solvent-free polymerizations were conducted in small glass vials filled with approximately 10 ml of reactants and two processing modes were applied (Figure 1a): Conventional, or silent, polymerizations using hot oil for the thermal pretreatment and processing of the reactants, or US assisted reactions for which an ultrasonic horn operating at 30 kHz was used in direct contact with the reactants. The reaction temperature was varied between 25 and 140 °C and the reaction progress was monitored based on the isocyanate consumption (decrease of the N=C=O stretching band at 2270 cm^-1) using FTIR (Figure 1d). The influence of the reactants stoichiometry was also studied, by varying the ratio of NCO / OH groups. The produced PUs were analyzed using various techniques which included GPC, FTIR, ¹H-NMR and DSC. The properties of the final PUs obtained by the two processing modes (Silent and US) were compared and the two processes were evaluated in terms of total energy requirements.

Results: A minimum temperature of 130-150 °C was found to be necessary for the non-catalyzed reactions between EG and HMDI. The effect of the stoichiometric ratio (in molar terms) of the reactants was confirmed to be detrimental for the final PUs molecular weight distribution (MWD) and heat capacity (Figure 1 b and c). Similarly, the polymeric chain architecture is also determined by the reactants ratio and quality (Figure 1e). When t,t HMDI reacted for example with EG, the -NH polyurethane linkage was altered. Both silent and US processing modes resulted in almost identical final products. Additionally, US application seems to decrease the total processing time considerably. Specifically, when sonication of 25 W electrical power (equivalent to 5 W calorimetric power) was applied for the reaction between HMDI and EG (1:2 molar ratio), ten minutes were required for total isocyanate consumption without any additional pretreatment requirements. For the conventional case of the same system, the processing time increased almost by a factor of three including total preheating and reaction time. Both mixing and heating benefits were noticed for the US application in the process. Improved mixing of the two phases formed by the reactants was achieved by complete emulsification of the bulk when sonication was applied. At the same time, depending on the US power applied, very fast heating of the bulk was possible. Experiments in the presence of catalyst revealed that the impact of temperature rise on the PUs production is not straightforward (for both conventional and US processing modes), but it seems that a temperature region exists where degradation of the products is possible. Specifically, at higher temperatures (100 °C) silent products were found to degrade, while degradation appeared at lower temperatures (50 °C) for the US produced polymers.

Conclusions and future steps: In this study, a rather challenging polymerization system is studied, and the reaction is activated using conventional or US treatment. The impact of the reactant quality and stoichiometry is explored, and the final products are characterized using various analytical techniques. The importance of the reactant ratio in the final products obtained is confirmed, and interesting observations are made related to the effect of the reaction temperature selection, when catalyst is present in the system. In addition, it is shown that US can be used to produce polymers of similar properties to the ones produced conventionally. Application of US facilitates both the mixing and the heating of the bulk, and for the reaction between EG and HMDI considerably shorter total processing time is required for the reaction completion. Future steps of this project include additional properties investigation for the PUs produced. Specifically, the final viscoelastic profile of the products will be studied. Further insights in the energy requirements of the two production methods are currently investigated.

References

1. Van Gerven, T. & Stankiewicz, A. Structure, energy, synergy, time-the fundamentals of process intensification. Ind. Eng. Chem. Res. 48, 2465–2474 (2009).
2. Sutkar, V. S. & Gogate, P. R. Design aspects of sonochemical reactors: Techniques for understanding cavitational activity distribution and effect of operating parameters. Chem. Eng. J. 155, 26–36 (2009).
3. Ashokkumar, M. The characterization of acoustic cavitation bubbles - An overview. Ultrason. Sonochem. 18, 864–872 (2011).
4. Suslick, K. S. & Price, G. J. Applications of Ultrasound to Materials Chemistry. Annu. Rev. Mater. Sci. 29, 295–326 (1999).
5. Sutkar, V. S., Gogate, P. R. & Csoka, L. Theoretical prediction of cavitational activity distribution in sonochemical reactors. Chem. Eng. J. 158, 290–295 (2010).
6. Price, G. J., Lenz, E. J. & Ansell, C. W. G. The effect of high intensity ultrasound on the synthesis of some polyurethanes. Eur. Polym. J. 38, 1531–1536 (2002).
7. Akindoyo, J. O. et al. Polyurethane types, synthesis and applications – a review. RSC Adv. 6, 114453–114482 (2016).