Role of Catalysts in the SPF Cell Formation Mechanism

Catalysts in the Cell Formation Mechanism of Rigid Polyurethane Foams Rigid polyurethane foams (RPFs), are formed by the expansion of material resulting from the reaction of a polymeric diisocyanate compound with a formulated polyol containing blowing agent, catalyst, surfactant and reinforcement agent(s) (clay, graphene, flame retardant, etc.) to 30-40 times its original volume (Akdoğan, 2023). Due to their closed cellular structure containing low thermal conductivity blowing gas molecules, RPFs are among the best thermal insulation materials in the world. Despite having different alternatives such as stone wool, polystyrene and lignocellulosic materials, RPFs are among the most fundamental materials used in thermal insulation due to their low thermal conductivity and low density properties. Additionally, the densely cross-linked structure of RPFs allows them to have higher mechanical strength at lower density compared to other conventional thermal insulation materials. When considering that one-third of global energy consumption in 2018 occurred during heating and cooling of buildings, the strategic importance of thermal insulation becomes clearly evident (IEA, 2020). The formation process of RPFs consists of four basic steps: dissolution of the blowing agent in the polyol, cell/bubble nucleation concurrent with the start of the polymerization reaction, growth of cells within the foam and stabilization of cell stability to bring the system into equilibrium (Verdolotti et al., 2017). For these steps to occur in the specified sequence, the selection of raw materials in the formulation is very important, and the type and amount of raw materials in the formulation are determinative regarding the foam's properties and consequently its application area. Although flexible and rigid polyurethane foams have fundamentally similar chemistry, differences in their properties are directly related to the characteristics of their constituent components, particularly polyols and isocyanates (Ashida, 2006; Defonseka, 2013). Figure 1 shows the development stages of cell morphology with changes in viscosity and temperature over time in an RPF formulation. Phase 1 is the stage in which polyol, surfactant, catalyst, blowing agent and reinforcement are mixed at high speed before the polymerization reaction begins. In Phase 2, cell nucleation begins with the addition of isocyanate. Two fundamental theories are accepted in the literature regarding cell nucleation. The first, known as classical nucleation theory, is based on the production of cells from the blowing agent following isocyanate addition. Due to the poor miscibility of the blowing agent with the polyol, a blowing agent microemulsion forms within the high-speed mixed reaction mixture. These small bubbles formed by the start of the polyaddition reaction begin to expand to form the final cells (Obi, 2018). The second theory is a newer theory based on the formation of air pockets through high-speed mixing of the mixture in Phase 1. In this case, the blowing agent migrates to the air pockets after the foam begins to swell (Reignier et al., 2019). Cell growth (Phase 3) is controlled by the viscosity and temperature of the mixture. Both of these parameters are related to the gelling and blowing steps adjustable by catalysts. The temperature increase after 30 seconds in Figure 1 results from the exothermic reaction of isocyanate with water or polyol. A small temperature increase is sufficient for the blowing agent to transition to the gas phase after the cream time. Simultaneous with cell growth, viscosity begins to increase because the cells act as viscosity modifiers (McClusky et al., 1994). Once cells begin to expand (around 60 s), a sudden drop in viscosity occurs. This is because the polymer heated by the exothermic reaction becomes more fluid. This drop in viscosity promotes the formation of cell walls and their contact with each other (Phase 4). Due to the foam's increasing viscosity and thermal insulation properties, temperature stabilizes around 70 s. The increasing amount of urea groups formed from the reaction of isocyanate and water is responsible for a second viscosity increase. Urea groups form strong hydrogen bonds with each other, creating hard segments of the polymer matrix (McClusky et al., 1994). The next viscosity drop observed around 100 s is explained by the delay between hard segment formation and gelling of the urethane network. Due to pressure differences in the cells, surface active agent causes drainage in the cell walls until polymer network structures form. Finally, the polyisocyanate and polyol reaction reaches the gelling point, viscosity becomes constant, and thus the foam morphology is fixed.

Catalysts

Catalysts that assist in polyurethane formation exert their effects through two potential mechanisms. The first is increasing the electrophilic character of carbon in the isocyanate group. The second is increasing the nucleophilic character of the active hydrogen-carrying molecule (Silva and Bordado, 2004). To control reaction kinetics in polyurethane foams, catalysts shown in Figure 2 are fundamentally preferred. Catalysts in polyurethane foams are responsible for three basic functions: i) formation of the urethane network structure, namely gelling, ii) formation of the reaction between water and isocyanate, namely blowing and iii) formation of isocyanurate rings (Van Maris et al., 2005). Tin salts such as tin octoate and dibutyltin dilaurate have higher gelling activity compared to sterically hindered amine compounds such as 1,4-diazabicyclo[2,2,2]octane (DABCO). Tertiary amines such as dibenzylamine, tetramethylethylenediamine and N,N-dimethylcyclohexylamine, which have less steric hindrance than DABCO, are classified as balanced catalysts because they catalyze both gelling and blowing reactions. Catalysts containing ether bonds close to the tertiary amine group, such as 2,2'-dimorfolinodietylether and 2,2'-oxybis(N,N-dimethylethane-1-amine), more predominantly catalyze blowing reactions. For the formation of isocyanurate rings, carboxylate-potassium salts catalyze the trimerization reaction in which isocyanate molecules actively participate in cross-linking (Figure 3). Besides these, new-generation multifunctional catalysts are also being developed with rapidly advancing technology. For example, in their study, Hamidov et al. (2021) synthesized a reactive type flame retardant with autocatalytic effect and examined its catalytic effect on hard-to-ignite properties. As a result, they stated that thanks to the material they synthesized, both the flame retardancy of the foam and catalytic activity (both gelling and blowing) increased. Cell growth is closely related to the balance between gelling and blowing catalysts. Modification of a component in the foam formulation (for example, using a different polyol) also changes the gelling/blowing balance. Figure 4 schematizes changes in temperature, viscosity and cell morphologies at different catalyst ratios. In the presence of high amounts of blowing catalyst, foam volume expands rapidly due to a sudden temperature increase (Figure 4a). For this reason, as shown in Figure 4a compared to Figure 4b, while cell size and elongation increase, foam density decreases. Moreover, rapid cell expansion causes cell wall tearing that results in open cells. In their study, Hakim et al. (2011) increased the blowing catalyst ratio in the foam formulation and detected decreases in foam densities from 41.41 kg/m³ to 30.01 kg/m³ with increasing blowing efficiency. Another effect is related to the increasing amount of blowing catalyst reducing the activity of the gelling catalyst. In this case, with the extended gelling time, the drainage time against pressure changes in the cells increases. Thus, cell walls tear, the closed cell ratio decreases and larger-celled foams are obtained. Conversely, when the amount of gelling catalyst is increased, the polymer solidifies faster than gas expansion. For this reason, foams with smaller cell size, higher closed cell ratio and higher density are obtained (Figure 4c). As a result, balancing these two catalyst types is necessary to obtain an optimal foam. Note: This mini review was prepared by citing Dr. Emre Akdoğan's doctoral thesis work.     References Akdoğan, E. (2023). Synthesis and optimization of rigid polyurethane foam from bioployols and formulation/structure/property relationships in foams. (Doctoral thesis, Eskişehir Technical University, Eskişehir, Thesis No: 787972) Ashida, K. (2006). Polyurethane and related foams: chemistry and technology. CRC press. Defonseka, C. (2013). Practical guide to flexible polyurethane foams. Smithers Rapra: Shawbury, UK. Hakim, A. A., Nassar, M., Emam, A., & Sultan, M. (2011). Preparation and characterization of rigid polyurethane foam prepared from sugar-cane bagasse polyol. Materials Chemistry and Physics, 129(1-2), 301-307. Hamidov, M., Çakmakçi, E., & Kahraman, M. V. (2021). Autocatalytic reactive flame retardants for rigid polyurethane foams. Materials Chemistry and Physics, 267, 124636. IEA, I. (2020). World Energy Balances: Overview. IEA Paris. Retrieved May, 19 from https://www.iea.org/reports/world-energy-balances-overview McClusky, J., O'Neill, R., Priester Jr, R., & Ramsey, W. (1994). Vibrating rod viscometer: a valuable probe into polyurethane chemistry. Journal of Cellular Plastics, 30(3), 224-241. Obi, B. E. (2018). 5-Fundamentals of Polymeric Foams and Classification of Foam Types. In Polymeric Foams Structure-Property-Performance (pp. 93-129). William Andrew Publishing. Peyrton, J., & Avérous, L. (2021). Structure-properties relationships of cellular materials from biobased polyurethane foams. Materials Science and Engineering: R: Reports, 145, 100608. Reignier, J., Alcouffe, P., Mechin, F., & Fenouillot, F. (2019). The morphology of rigid polyurethane foam matrix and its evolution with time during foaming–New insight by cryogenic scanning electron microscopy. Journal of colloid and interface science, 552, 153-165. Silva, A. L., & Bordado, J. C. (2004). Recent developments in polyurethane catalysis: catalytic mechanisms review. Catalysis reviews, 46(1), 31-51. Van Maris, R., Tamano, Y., Yoshimura, H., & Gay, K. M. (2005). Polyurethane catalysis by tertiary amines. Journal of Cellular Plastics, 41(4), 305-322. Verdolotti, L., Di Caprio, M. R., Lavorgna, M., & Buonocore, G. G. (2017). Polyurethane nanocomposite foams: Correlation between nanofillers, porous morphology, and structural and functional properties. In Polyurethane Polymers (pp. 277-310). Elsevier.   Dr. Emre Akdoğan Research Assistant Department of Chemistry Eskişehir Technical University   Prof. Dr. Murat Erdem Department of Chemistry Eskişehir Technical University