Latest Developments in Polymer Applications

Recent Developments in Polymer Applications Reinforced with Hybrid Flame Retardants Polymers are materials we frequently use in our daily lives due to their electrical and thermal properties (conductive/insulating), optical, acoustic and other characteristics, as well as their low density, superior physicochemical properties and chemical modifiability. However, the rapid and easy flammability of these materials is their greatest disadvantage. Additionally, combustion can release smoke and toxic gases to the atmosphere that are extremely hazardous to living organisms and ecological systems. Fires originating from or containing polymers cause significant loss of life and property worldwide each year. According to recent research conducted by Shi et al. (2021) [1], fires resulting from easily flammable polymeric materials cause more than 40,000 deaths globally, and total fire losses constitute approximately 1% of a country's gross domestic product (GDP). For this reason, improving fire safety for polymeric materials has become increasingly mandatory. In this context, many countries have established laws and regulations to require polymeric materials to be given flame-resistant properties. The development of flame-resistant polymers required by these laws and regulations is both necessary and unavoidable [2]. Adding flame retardants to polymers during manufacturing is one of the most effective, practical and economical ways to improve their flame-resistant properties. Flame retardants can be classified into three categories based on their composition: organic, inorganic and organic-inorganic hybrid flame retardants. Inorganic flame retardants such as metal oxides, metal hydroxides and clays are used to impart flame-resistant properties to polymers due to their easy availability, non-flammable characteristics and high heat capacity [5, 6]. To achieve high flame-resistant performance with these types of flame retardants, they must be added in high quantities to the polymer matrix. This causes deterioration of mechanical and thermal properties due to poor compatibility between inorganic materials and polymers. Organic flame retardants provide high flame resistance and are generally compatible with polymers. In recent years, organophosphorus flame retardants have been intensively studied for their flame-resistant properties, and these types of compounds are used as flame retardants in many industrial polymer products [7]. However, these halogen-free organic flame retardants carry the risk of leaching from the polymer matrix over time and polluting the environment [8]. Additionally, the low thermal stability of organic flame retardants can damage polymers obtained at high temperatures. Today, organic-inorganic hybrid flame retardants have attracted considerable interest. This is because these flame retardants combine all the individual advantages and properties of organic and inorganic flame retardants, enabling them to exhibit superior flame-resistant performance. In a hybrid flame retardant system, the organic component contributes to char layer formation while the inorganic component provides thermal stability to the resulting char layer [9]. Moreover, when hybrid flame retardants are added to polymers, the physicochemical properties of polymers generally improve due to additive/matrix compatibility and uniform dispersion of the additive within the matrix. There are four fundamental approaches to synthesizing hybrid flame retardants: i) covalent bonding, ii) ion combination, iii) hydrogen bonding and iv) π-π interaction [10]. The most commonly used method for creating hybrid flame retardants is the covalent bonding method. There are many different methods for synthesizing hybrid flame retardants using the covalent bonding approach: 1) Hybrid flame retardants can be readily synthesized by reacting inorganic nanoparticles containing reactive functional groups such as hydroxyl, epoxy, amino and/or carboxyl groups with organic flame retardants containing similar groups. For example, hybrid flame retardants have been synthesized by attaching different organic flame retardants via covalent bonds to many inorganic flame retardants such as expandable graphite (EG) [11], aluminum hydroxide (ATH) [12] (Figure 1) and zinc oxide (ZnO) [13]. 2) If the organic and inorganic flame retardant do not have a reactive end group, the surface of the inorganic flame retardant is modified to create reactive sites, and subsequently hybrid flame retardants can be synthesized. 3) The inorganic phase can be synthesized in situ within the hybrid flame retardant system. The sol-gel method is the most preferred approach. [caption id="attachment_130666" align="aligncenter"] Figure 1. ATH-containing hybrid flame retardant reaction [12][/caption] Combining ions through electrostatic interactions is another important method used for preparing hybrid flame retardants. Various inorganic-organic hybrid flame retardants have been produced through cation exchange between ammonium polyphosphate (APP) and diethylenetriamine [14] or anion exchange between phosphomolybdic acid and phosphonate-based ionic liquid [15]. Additionally, in recent years, different hybrid flame retardants with varying properties have been created using layer-by-layer coating with ions [16]. Through hydrogen bonding interactions, SiO2 nanosphere/graphene oxide (GO) hybrid flame retardants are generally obtained. The -OH and -COOH groups in the GO structure form hydrogen bonds with similar groups on the SiO2 nanosphere surface, creating a SiO2 nanosphere/GO hybrid flame retardant system [17]. In addition to the three strategies mentioned above, π-π interaction is a new approach used for obtaining hybrid flame retardants. Since this method requires abundant aromatic structures in both phases, it is rarely used. Regarding the application of hybrid flame retardants to rigid polyurethane foam (RPUF), an important thermal insulation material, it would be beneficial to examine two studies recently published in the literature. Xu et al. (2019) [18] prepared a core/shell structured ZIF-8@MA by coating ZIF-8 with melamine, and subsequently synthesized a ternary hybrid flame retardant designated as ZMD using ZIF-8@MA and diatomite (Figure 2). They then added their synthesized hybrid flame retardant to RPUF at 10% by mass and examined its combustion performance properties. Cone calorimeter results showed that ZMD-modified RPUF achieved a 50.1% reduction in peak heat release rate (pHRR), 61.8% reduction in total heat release (THR), 70.6% reduction in smoke production rate (SPR) and 76.1% reduction in total smoke release (TSR) compared to unmodified foam. The limiting oxygen index value (LOI) increased from 19.4% to 25.4% with the addition. Through tests conducted to understand the flame-retardant mechanism, they demonstrated that melamine works in the gas phase, ZnO formed by ZIF-8 decomposition and silica in diatomite work in the solid phase. [caption id="attachment_130667" align="aligncenter"] Figure 2. ZMD hybrid flame retardant synthesis [18][/caption] Yuan and colleagues in their 2020 study [19] synthesized Cu2O nanoparticles between molybdenum disulfide (MoS2) nanolayers (Figure 3) and added this structure to RPUF at only 1% by mass. Significant improvements were found in both physicochemical and flame-resistant properties of the hybrid flame retardant-modified RPUF. The addition provided significant improvements in the foam's smoke suppression properties; hydrogen cyanide (HCN), nitrogen oxide (NOx) and carbon monoxide (CO) emissions from the composite foam showed reductions of 15.4%, 53.3% and 28% respectively compared to unmodified foam. They emphasized that these reductions were due to physical adsorption by MoS2 and the catalytic effect of Cu2O. [caption id="attachment_130669" align="aligncenter"] Figure 3. Cu2O-MoS2 hybrid flame retardant synthesis [19][/caption] Hybrid flame retardant technology has been widely used in preparing polymer composites over the past decade due to its flame-retardant efficiency, ability to enhance physicochemical properties and multifunctionality. Unlike conventional flame retardants, most hybrid flame retardants provide significant reductions in heat release rate (typically >30%) at low additive loadings (typically <5%). Additionally, conventional flame retardants only help suppress heat release rates during combustion without accounting for toxic gases and smoke released from polymeric materials. However, in fire situations, the primary cause of human deaths is not the heat generated but the toxic gases and suffocating smoke produced. Hybrid flame retardants offer promising solutions for developing polymeric materials with superior flame-resistant properties. In case of fire, one component in the hybrid system reduces flame spread and heat release while the other helps reduce the emission of toxic gases and smoke, thereby preventing potential loss of life and property. Despite the many superior properties of hybrid flame retardants, there are still some characteristics that need to be questioned, researched and developed: i) The number of studies investigating the synergistic effects of the quality and quantity of organic and inorganic phases in hybrid flame retardants on the flame-resistant properties of polymer composites/nanocomposites is quite limited. ii) Although these materials significantly reduce dangerous gas and smoke emissions, the mechanisms of toxic gas conversion and smoke suppression remain unclear. Therefore, some advanced characterization technologies are needed to clarify these mechanisms. iii) Commercialization of hybrid flame retardants is limited by high costs and complex technological manufacturing processes; therefore, there is still a long way to go before moving polymer composites containing such materials into production. Despite these challenges, hybrid flame retardants will face increasing demand due to the flame resistance, low heat release, reduced toxic gas and smoke emissions they provide to polymers without compromising physicochemical properties. This demand will drive increased research and development activities in hybrid flame retardant technologies by both academia and industry.

Emre Akdoğan Research Assistant Eskişehir Technical University, Faculty of Science, Department of Chemistry

Acknowledgment

I express my thanks to my thesis advisor Prof. Dr. Murat Erdem for his contributions. References [1] Y. Shi, B. Yu, X. Wang, A.C.Y. Yuen, Flame-Retardant Polymeric Materials and Polymer Composites, Frontiers in Materials 8 (2021) 195. [2] Y. Hou, Z. Xu, F. Chu, Z. Gui, L. Song, Y. Hu, W. Hu, A review on metal-organic hybrids as flame retardants for enhancing fire safety of polymer composites, Composites Part B: Engineering (2021) 109014. [3] E. Akdogan, M. Erdem, M.E. Ureyen, M. Kaya, Synergistic effects of expandable graphite and ammonium pentaborate octahydrate on the flame-retardant, thermal insulation, and mechanical properties of rigid polyurethane foam, Polymer Composites 41(5) (2020) 1749-1762. [4] E. Akdogan, M. Erdem, M.E. Ureyen, M. Kaya, Rigid polyurethane foams with halogen-free flame retardants: thermal insulation, mechanical, and flame retardant properties, Journal of Applied Polymer Science 137(1) (2020) 47611. [5] P. Kiliaris, C. Papaspyrides, Polymer/layered silicate (clay) nanocomposites: an overview of flame retardancy, Progress in polymer science 35(7) (2010) 902-958. [6] A. Dasari, Z.-Z. Yu, G.-P. Cai, Y.-W. Mai, Recent developments in the fire retardancy of polymeric materials, Progress in Polymer Science 38(9) (2013) 1357-1387. [7] N.A. Isitman, C. Kaynak, Nanoclay and carbon nanotubes as potential synergists of an organophosphorus flame-retardant in poly (methyl methacrylate), Polymer Degradation and Stability 95(9) (2010) 1523-1532. [8] J. Andresen, A. Grundmann, K. Bester, Organophosphorus flame retardants and plasticisers in surface waters, Science of the total environment 332(1-3) (2004) 155-166. [9] C.K. Kundu, L. Song, Y. Hu, Multi elements-based hybrid flame retardants for the superior fire performance of polyamide 66 textiles, Journal of the Taiwan Institute of Chemical Engineers 118 (2021) 284-293. [10] X. Wang, W. Guo, W. Cai, J. Wang, L. Song, Y. Hu, Recent advances in construction of hybrid nano-structures for flame retardant polymers application, Applied Materials Today 20 (2020) 100762. [11] X. Chen, J. Zhuo, W. Song, C. Jiao, Y. Qian, S. Li, Flame retardant effects of organic inorganic hybrid intumescent flame retardant based