Recent Advances in Bio-Based Flame Retardant Additives for Synthetic Polymeric Materials
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polymers Review Recent Advances in Bio-Based Flame Retardant Additives for Synthetic Polymeric Materials Christopher E. Hobbs Department of Chemistry, Sam Houston State University, Huntsville, TX 77340, USA; [email protected]; Tel.: +1-936-294-3750 Received: 28 December 2018; Accepted: 24 January 2019; Published: 31 January 2019 Abstract: It would be difficult to imagine how modern life across the globe would operate in the absence of synthetic polymers. Although these materials (mostly in the form of plastics) have revolutionized our daily lives, there are consequences to their use, one of these being their high levels of flammability. For this reason, research into the development of flame retardant (FR) additives for these materials is of tremendous importance. However, many of the FRs prepared are problematic due to their negative impacts on human health and the environment. Furthermore, their preparations are neither green nor sustainable since they require typical organic synthetic processes that rely on fossil fuels. Because of this, the need to develop more sustainable and non-toxic options is vital. Many research groups have turned their attention to preparing new bio-based FR additives for synthetic polymers. This review explores some of the recent examples made in this field. Keywords: flame retardant; bio-based; green chemistry 1. Introduction Although instilling flame resistance has been a subject of interest since the ancient Egyptians [1], “modern” chemists have only been on the case since the 18th and 19th centuries, as evidenced by the words of American poet Emily Dickinson [2]: “Ashes denote that fire was—revere the grayest pile, for the departed creature’s sake, that hovered there awhile—fire exists the first in light, and then consolidates, only the chemist can disclose, into what carbonates.” The same era saw the publication of landmark work by French chemist Joseph Louis Gay-Lussac (who studied flame resistance with the intent of protecting theatres) and Scottish chemist M. M. Pattison Muir, who published The Chemistry of Fire in 1893, only a few years after Emily Dickinson’s death [3–5]. In the century-and-a-half since, our understanding of the molecular world, as it pertains to fire, has progressed significantly and the scientific community has made tremendous advances so that chemists can fully disclose into what carbonates. The 20th century saw enormous advances in the chemical sciences. In particular, the rise of polymer chemistry led to a revolution in daily life across the industrialized world. Because of this, these synthetic materials (e.g., plastics and fibers) have become ubiquitous. However, the extremely high levels of flammability often exhibited by these materials is a consequence of their daily use. But, the advances made in polymer chemistry led to an increase in our understanding of fire resistance and provided chemists the motivation to study and develop better flame resistant (FR) polymers. Early commercial examples include: Bakelite, Nomex, Kevlar, and others. [3] This is understandably an important topic of industrial and academic research considering that structure fires cause $6.7 billion worth of damage, 12,300 injuries, and >2500 deaths annually in the United States alone [6]. The massive California wild fires in the summer and autumn of 2018 were a stark reminder of the threat that fires pose. For these reasons, the number of reports detailing the preparation of new FR polymeric materials has exploded in recent years (Figure1). Polymers 2019, 11, 224; doi:10.3390/polym11020224 www.mdpi.com/journal/polymers Polymers 2018, 10, x FOR PEER REVIEW 2 of 30 threat that fires pose. For these reasons, the number of reports detailing the preparation of new FR Polymers 2019, 11, 224 2 of 31 polymeric materials has exploded in recent years (Figure 1). Figure 1.1. GraphGraph representing representing the the number number of peer-reviewed of peer-revie journalwed journal articles (foundarticles in (found SciFinder) in SciFinder) describing flamedescribing resistant flame (FR) resistant polymers (FR) per polymers year between per year 1969 between and 2017. 1969 and 2017. This growth in the number of reports of new FR materials has been spurred by advances in science This growth in the number of reports of new FR materials has been spurred by advances in as well as closer scrutiny by governmental regulatory bodies, many of which have sought to decrease science as well as closer scrutiny by governmental regulatory bodies, many of which have sought to and outright ban the use of halogenated FR materials in the last few decades [7]. In light of this, decrease and outright ban the use of halogenated FR materials in the last few decades [7]. In light of it has been the responsibility of the scientific community to develop new, safer alternatives to these this, it has been the responsibility of the scientific community to develop new, safer alternatives to traditional FR systems, of which phosphorus-containing species are some of the most studied [8]. To do these traditional FR systems, of which phosphorus-containing species are some of the most studied so, it is important to understand how polymeric materials burn: combustion of polymeric material [8]. To do so, it is important to understand how polymeric materials burn: combustion of polymeric involves evolution of combustible volatiles through decomposition in an oxygen-rich atmosphere. material involves evolution of combustible volatiles through decomposition in an oxygen-rich No matter the type of FR used, the mechanisms for flame retardancy occur by one of two routes (or a atmosphere. No matter the type of FR used, the mechanisms for flame retardancy occur by one of combination of the two): (i) condensed phase or (ii) vapor phase [9]. Flame resistance/retardancy in two routes (or a combination of the two): (i) condensed phase or (ii) vapor phase [9]. Flame the condensed phase often involves the formation of char layer that can stop the spread of the flame resistance/retardancy in the condensed phase often involves the formation of char layer that can stop and self-extinguish, protecting the material by limiting the transfer of heat and oxygen to the polymer. the spread of the flame and self-extinguish, protecting the material by limiting the transfer of heat Vapor phase flame resistance/retardancy involves terminating the radical chemistry involved in fire, and oxygen to the polymer. Vapor phase flame resistance/retardancy involves terminating the radical often the mechanism by which traditional halogenated FR materials operate (Table1). chemistry involved in fire, often the mechanism by which traditional halogenated FR materials operate (Table Table1). 1. Mechanisms of flame retardancy based on phase and example of each. PhaseTable 1. Mechanisms of flame retardancy Method based on phase and example of Example each. 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