Hybrid Flame Retardants

Recent Advances in Hybrid Flame Retardants Reinforced Polymer Applications Polymers are materials which have been widely utilized in our daily life due to their electrical and thermal (conductive/insulator), optical and acoustic properties, as well as low density, superior physico-mechanical and chemically modifiable properties. However, the polymers are eager to be ignited quickly and easily. Furthermore, during the burning of polymers, a large number of toxic gases and smoke that are extremely harmful to humans and the environment are released. Consequently, polymer fires cause a large number of losses of life and properties every year worldwide. According to the last research done by Shi et al. (2021) [1], over 40,000 deaths have taken place as a result of fires originating from the flammable polymeric materials worldwide and total fire loses have accounted for around 1% of the gross domestic product (GDP) of the country. Therefore, fire safety requirement on polymers is becoming increasingly necessary day by day. In this context, lots of countries have started to create new laws and regulations to make sure that the incorporation of flame retardant is mandatory for polymeric materials. The development of fireproof polymers forced by these laws and regulations is necessary and inevitable [2]. The introduction of flame retardant to polymers during the production process is the most effective, practical, and economical way in order to improve fire safety characteristics [3, 4]. According to the component of flame retardants, they can be divided into three types: organic, inorganic, and organic-inorganic hybrids. Inorganic flame retardants such as metal oxides, metal hydroxides, and clays are used to fabricate fireproof polymers due to their accessibility, incombustibility, and high specific heat capacity [5, 6]. To obtain a polymer with a high fire resistance using these flame retardants, it is necessary to incorporate a huge amount of flame retardant into the polymer matrix. The use of a high amount of flame retardants causes poor mechanical and thermal properties because of the low compatibility between polymer and inorganic flame retardants. Organic flame retardants provide high flame retardancy and almost there are no compatibility problems with polymers. In the last decades, the flame retardant properties of organophosphorus compounds are studied intensively and used practically in many industrial polymer products [7]. However, these halogen-free organic flame retardants possess the potential of separating from polymer matrix in time, thus polluting environments [8]. In addition, their poor thermostability can be harmful to the polymers processing at high temperatures. Nowadays, organic-inorganic hybrid flame retardants are attracting considerable attention because these flame retardants combine all the individual advantages/ properties of organic and inorganic flame retardants, resulting that the polymers reinforced with hybrid flame retardants display superior flame retardant performance. In the hybrid flame retardant systems, the organic portion helps the formation of char, while the inorganic portion strengthens the formed char layer [9]. Besides, the addition of hybrid flame retardants to the polymer improves the physico-mechanical properties owing to the excellent compatibility and homogenous dispersion in the polymer matrix. Basically, there are four basic approaches to synthesizing hybrid flame retardants: i) covalent linkage, ii) ionization assembly, iii) hydrogen bonding and iv) π-π interaction [10].

Covalent linkage is the most preferred strategy to fabricate hybrid flame retardants. There are lots of different methods to prepare hybrid flame retardants using covalent linkage strategy:

1) Inorganic nanoparticles possessing hydroxyl, epoxy, amino, and/or carboxyl groups could easily give a reaction with organic flame retardant having active functional groups like hydroxyl, amino, and so forth. For instance, various organic flame retardants were covalently synthesized by grafting on inorganic flame retardants such as expandable graphite (EG) [11], aluminum hydroxide (ATH) [12] (Figure 1), and zinc oxide (ZnO) [13]. 2) If organic and/or inorganic flame retardants have no proper active sites, the reactive sites could be created by modifying the surface of flame retardants; and afterward, hybrid flame retardants can be synthesized. 3) Inorganic phase can in situ be formed in hybrid flame retardant. The sol-gel method is the most used method to prepare a new inorganic phase in hybrid flame retardant. [caption id="attachment_133623" align="aligncenter" width="516"] Figure 1. The reaction of hybrid flame retardant containing ATH [12][/caption]Ionization assembly is another attractive method to produce hybrid flame retardant through the attraction of positive/negative charges. Several organic-inorganic hybrid flame retardants were prepared via cation exchange between ammonium polyphosphate (APP) and diethylenetriamine [14], or anion exchange between phosphomolybdic acid and phosphonate-based ionic liquid [15]. Moreover, in the last decades, many hybrid flame retardants were fabricated via layer-by-layer ionization assembly [16]. Hydrogen bonding interaction is generally used for fabricating SiO₂ nanosphere/graphene oxide (GO) hybrid flame retardant. The functional groups such as -OH and -COOH of GO could form hydrogen bonding interaction with -OH groups on the surface of the SiO₂ nanosphere. Then, SiO₂ nanosphere/GO hybrid flame retardant could be self-assembled [17]. In addition to these mentioned strategies, π-π interaction is a newly used approach to fabricate hybrid flame retardants. This method is rarely used because both inorganic and organic phases have to have plenty of aromatic structures in order to produce hybrid flame retardant. It would be useful to examine two of the recent studies on the application of hybrid flame retardants to rigid polyurethane foam (RPUF) which is an important thermal insulation material. Xu et al. (2019) [18] prepared the core-shell structure (ZIF-8@MA). Firstly, melamine (MA) was coated with zeolitic imidazolate framework-8 (ZIF-8), and the ternary composite ZMD containing Si-N-Zn was successfully synthesized with the diatomite modified ZIF-8@MA (Figure 2). Afterward, the synthesized hybrid flame retardant was incorporated into RPUF in the amount of 10% by weight and they examined the fire performance properties of RPUF composites. According to cone calorimeter test results, when compared to that of neat RPUF, the peak heat release rate (pHRR), total heat release (THR), smoke production rate (SPR), and total smoke production (TSP) of RPUF having ZMD decreased by 50.1%, 61.8%, 70.6%, and 76.1%, respectively. The value of limiting oxygen index (LOI) increased up to 25.4% from 19.4%. Concerning the results done in order to explore the flammability mechanism, they proved that MA is a gas phase active flame retardant while ZnO formed by the decomposition of ZIF-8 and silica in diatomite catalyzed the formation of char in the condensed phase. [caption id="attachment_133624" align="aligncenter" width="437"] Figure 2. Synthesis scheme of ZMD-based hybrid flame retardant [18][/caption]Yuan et al. (2020) [19] synthesized copper (I) oxide (Cu₂O) nanoparticles between the layers of molybdenum disulfide (MoS₂) (Figure 3). The obtained hybrid flame retardant was added to the RPUF and the loading was only maintained at 1 wt%. It was determined that both physo-mechanical and flame retardancy properties of RPUF with hybrid flame retardant increased significantly. In addition, the amounts of the toxic gases were decreased greatly. When compared to that of neat RPUF, the amounts of hydrogen cyanide (HCN), nitrogen oxides (NOx), and carbon monoxide (CO) were reduced by 15.4%, 53.3%, and 28%, respectively. The reason behind these decrements is the synergistic effect between the physical adsorption of MoS2 and the catalysis action of Cu₂O. [caption id="attachment_133625" align="aligncenter" width="603"] Figure 3. Synthesis scheme of Cu2O-MoS2 hybrid flame retardant [19][/caption]The technology of hybrid flame retardant has been tried to be improved on fabricating polymer composites in the last decades due to its high fire resistance, reinforcing effect, and multi-functionality. Contrary to traditional flame retardants, a relatively low loading (generally < 5%) of most hybrid flame retardants could result in a significant decrease of heat release rate (mostly > 30%). Furthermore, traditional flame retardants solely try to suppress the heat release rate without taking into account the toxic gases and smoke emission from polymeric materials during burning. However, toxic gases and smoke rather than heat release are the primary reason for the loss of life. Hybrid flame retardant technology proposes a promising solution for the development of polymeric materials with superior properties against fire risk. In the case of fire, while one of the components of hybrid flame retardant decreases the flame propagation or heat release, the other component lowers toxic gases and smoke emissions, attempting to prevent the potential loss of life and possessions caused by fire hazards. Even though hybrid flame retardants have several superior properties, there are still lots of challenges that need to be questioned, researched, and developed: i) The number of studies quantitatively investigating the synergistic effect of the component ratios of organic and inorganic phases in hybrid flame retardants on the flame retardancy of polymer composites/nanocomposites is very few. ii) Although a great amount of poisonous volatile gases reduces, the conversion of toxic gases and smoke suppression mechanism are not clear. Therefore, some advanced characterization techniques are needed to explore these mechanisms. iii) The commercialization of hybrid flame retardants is limited due to the high production cost and processing technology and there is still a long way to put the production of polymer composites reinforced with hybrid flame retardant into practice. Despite these unresolved problems, these hybrid flame retardants will be demanded more and more in the area of fire safety polymers due to the properties including difficult ignition, low heat release rate, low toxic gases, and smoke emission without damaging the physico-mechanical properties. This demand will further increase the research and development activities of both academia and industry for hybrid flame retardant technologies.

Acknowledgements

I would like to show my appreciation to my supervisor, 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 on expandable graphite in silicone rubber composites, Polymers for advanced technologies 25(12) (2014) 1530-1537. [12] Y. Cao, Y. Ju, F. Liao, X. Jin, X. Dai, J. Li, X. Wang, Improving the flame retardancy and mechanical properties of poly (lactic acid) with a novel nanorod-shaped hybrid flame retardant, RSC advances 6(18) (2016) 14852-14858. [13] B. Xu, X. Wu, W. Ma, L. Qian, F. Xin, Y. Qiu, Synthesis and characterization of a novel organic-inorganic hybrid char-forming agent and its flame-retardant application in polypropylene composites, Journal of analytical and applied pyrolysis 134 (2018) 231-242. [14] Y. Tan, Z.-B. Shao, X.-F. Chen, J.-W. Long, L. Chen, Y.-Z. Wang, Novel multifunctional organic–inorganic hybrid curing agent with high flame-retardant efficiency for epoxy resin, ACS applied materials & interfaces 7(32) (2015) 17919-17928. [15] F. Xiao, K. Wu, F. Luo, S. Yao, M. Lv, H. Zou, M. Lu, Influence of ionic liquid-based metal–organic hybrid on thermal degradation, flame retardancy, and smoke suppression properties of epoxy resin composites, Journal of Materials Science 53(14) (2018) 10135-10146. [16] K.M. Holder, R.J. Smith, J.C. Grunlan, A review of flame retardant nanocoatings prepared using layer-by-layer assembly of polyelectrolytes, Journal of Materials Science 52(22) (2017) 12923-12959. [17] J. Liu, R.K. Yuen, N. Hong, Y. Hu, The influence of mesoporous SiO2-graphene hybrid improved the flame retardancy of epoxy resins, Polymers for Advanced Technologies 29(5) (2018) 1478-1486. [18] W. Xu, G. Wang, J. Xu, Y. Liu, R. Chen, H. Yan, Modification of diatomite with melamine coated zeolitic imidazolate framework-8 as an effective flame retardant to enhance flame retardancy and smoke suppression of rigid polyurethane foam, Journal of hazardous materials 379 (2019) 120819. [19] Y. Yuan, W. Wang, Y. Shi, L. Song, C. Ma, Y. Hu, The influence of highly dispersed Cu2O-anchored MoS2 hybrids on reducing smoke toxicity and fire hazards for rigid polyurethane foam, Journal of hazardous materials 382 (2020) 121028. Emre Akdoğan Research Asistant Eskisehir Technical University, Faculty of Science, Chemistry Department   .