Cell Openers in Polyurethane Foam Systems

Effects of Cell Openers Used in Polyurethane Foam Systems

Introduction

Polyurethane is formed as a result of an exothermic reaction when isocyanates containing free Nitrogen-Carbon-Oxygen (NCO) bonds combine with polyols containing hydroxyl (OH-1) groups. In this reaction, the Hydrogen (H) atom from the hydroxyl group in polyols bonds to the Nitrogen (N) atom found in isocyanates. Polyurethane materials can be produced with different physical properties depending on their field of application. Generally, there are two different structures: soft (flexible) and rigid (hard). Polyurethanes synthesized with diisocyanates and diols are thermoplastic polymers with linear chain structures. When the functionality properties of isocyanate or alcohol are increased, cross-linked thermoset polyurethanes are obtained. By controlling the number of functional groups actively involved in the reaction, it is possible to produce polyurethane with different cross-linked structures ranging from flexible to rigid. Additionally, when water, which has blowing properties, enters the reaction with isocyanate, carbon dioxide (CO2) gas is released. As this gas escapes from the environment, it leaves behind a porous structure and makes the polyurethane porous. At the same time, small sub-formations called nuclei are created in the foundation of the polymer structure. During the reaction, these nucleus structures expand and in the final stage create a stable porous structure. In rigid polyurethanes, the pore structures are closed, whereas in soft systems the pore structures are open. In rigid foams with closed pore structures, it is expected that 85-95% of the pore structure will be closed. In parallel, in soft polyurethane foams, the open pore structure is similarly quite high. As can be understood from this, it is not possible for polyurethane foam systems to have entirely closed or open pore structures. The size and homogeneity of the pores present in the product are important factors affecting the product's properties. Structures with visibly apparent pore structures are called macro-porous polyurethane foam, while structures with pores too small to be seen with the naked eye are called micro-porous polyurethane. One of the factors affecting pore structure in polyurethane foams is the use of different raw materials. Likewise, the production technology used also affects pore structure. As mentioned above, if the polyurethane foam to be created will be in thermoplastic form, the liquid polymer mixture is first foamed and then shaped and mixed using an appropriate method. If thermoset polyurethane foam is desired, resins containing prepolymer along with additives are foamed. Subsequently, it is solidified in a mold with a cross-linked structure. Polyurethane foams also differ according to their fields of application. Depending on their pore structures, they are used in thermal insulation, sound insulation, energy absorption, the automotive sector, healthcare, textile leather production, and in upholstery such as pillows and mattresses, as well as in different areas such as shoes, slippers, and sponges. To impart the desired physical properties according to the field of application, shaping the pore structures (cell structures) in polyurethane sponge forms is one of the important criteria. For this purpose, it is possible to use additives called cell openers from outside and to expand the cell walls. By expanding the cell walls and changing the distance between cells, it is possible to provide air passage and controlled release of CO2 gas generated during the reaction. In this way, during the formation of polyurethane sponge forms, balanced cell structure formation is ensured. Because if cells formed during polymerization are not enlarged in a controlled manner, cell coalescence occurs and, as mentioned above, gas entry and exit cannot be effectively provided. This causes structural collapse. To prevent this, using cell opener will help eliminate problems such as inward contraction, collapse, and cracking of the structure. Thus, a more flexible and softer polyurethane material is created. In Figure 1, using a ZEISS EVO LS 10 Scanning Electron Microscope (SEM), the pore structures of polyurethane sponge systems created by adding cell opener in different ratios were examined at four (4) different magnifications: 50, 100, 250, and 500. In Figure 1, the 50-fold magnified SEM images of polyurethane foams containing no cell opener, 1 part, 2 part, 3 part, 4 part, and finally 5 part cell opener are shown in sequence. These were named Test 0, Test 1, Test 2, Test 3, Test 4, and Test 5 respectively. As seen in Figure 1, the polyurethane foam system without addition is the tightest structure with the least space between cells. When the amount of cell opener is added in parts, the most cell opening or highest porosity is found in the polyurethane foam system with 5 parts added. Since the cell membrane in the cell structure is more flexible, CO2 gas release is expected to be easier. Thus, the amount of contraction will be less compared to the others.

Conclusion

Polyurethane systems are formed as a result of an exothermic reaction occurring when isocyanates and polyols come together. As a result of the reaction between isocyanates and polyols containing hydroxyl groups, carbon dioxide (CO2) gas is released. As this gas is expelled from the system and the resulting space is filled with outside air, a porous structure is created. Controlled shaping of this porous structure affects the physical properties of the desired final product. For this reason, polyurethane is produced by externally adding additives called cell openers. As seen in this study, while the cell opener additive expands the pore structure, it makes the cell membrane more flexible, enabling CO2 gas to be more easily expelled. This also contributes to the system being more flexible.  

Acknowledgment

The authors thank Adıyaman Üniversitesi Rectorate, Vice Rector Prof. Dr. Cumhur Kırılmış, and Elif Pekeroğlu Temurtaş, Scanning Electron Microscope Manager at the central laboratory, for their assistance in obtaining images with the Scanning Electron Microscope.   References [1] Erkin Akdoğan, "Effects of Different Additives on Mechanical Properties of Polyurethane Materials, Master's Thesis, Mechanical Engineering, Pamukkale Üniversitesi, 2011" [2] Yasemin Karel, "Vegetable Oil-Based Polyol" [3] https://polen.itu.edu.tr/items/aa959655-501c-4ccd-a681-f0b9dbd563b3 [4]http://www.plastik-ambalaj.com/tr/hakkimizda/kurumsal/119-plastik-ambalaja-makale/2699-polimerik-koepuekler-koepuek-morfolojisi [5] Sami Erol, "Flexible Polyurethane Sponge Production Technology and Modelling of Temperature Distribution in the Production System" Anadolu Üniversitesi, Master's Thesis, 1988   Dr. Özden Demircioğlu Research and Development Center Manager Baymel Kimya Yapı A.Ş.   Erhan Altıntuğ Research and Development Engineer Baymel Kimya Yapı A.Ş.