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Before beginning any book on non-halogenated flame retardants, it is important to understand the history of why there is such a book focused on non-halogenated flame retardants. Prior to the late 20^th century, flame retardants were not necessarily singled out by any particular chemistry. Indeed, prior to the 1930s, halogen was not used at all as a flame retardant chemical, and even after its discovery and use, it was just another class of chemicals used to impart fire safety to other materials.

To begin with, a flame retardant chemical is a chemical that shows the ability to retard flame growth and spread in a particular material in a particular fire risk scenario. This flame retarding function can be achieved with very diverse chemistries based on the elements bromine, chlorine, phosphorus, nitrogen, and aluminum to name the most prominent. Many of these elements, other than halogen, are discussed throughout this book. On top of these elements, both organic and inorganic substances are being used. The only common feature is that the flame retardant interferes with some of the chemical reactions which are necessary for sustained burning of a material and generally raises the energy that is necessary to ignite a material – flame retardants do not make materials non-combustible.

Not all flame retardants are universally able to flame retard all polymers in all fire risk scenarios. A particular chemical may be very effective in one polymer, but not in another. This is really no different than most chemicals in use throughout the world today: each has its specific chemistry it is capable of, and its own chemical structure-property relationships that yield certain end effects when a chemical reaction occurs. There can be simplicity in grouping chemicals by general structural class and similarity due to how they chemically react. For example, halogenated flame retardants tend to have very similar flame retardant mechanisms of vapor phase combustion inhibition, regardless of chemical structure. There are exceptions where aliphatic and aromatic halogenated compounds can have different reactivity in fire events, as well as additional fuel/chemical interactions that one class will show and the other will not, but some general mechanisms of flame retardancy can be assigned to a group of similar chemicals. As will be discussed, some general classes of flame retardant chemicals include halogenated, phosphorus-based, mineral fillers, nitrogen-based, silicon-based, boron-based, and a wide range of other niche chemicals ranging from transition metal materials to metalloids to carbon-based structures. So while it is possible to group chemicals by flame retardant activity and mechanism, it becomes more complicated to group those same chemicals for reactivity in non-flame retardant scenarios. For example, one mineral filler used as a flame retardant, magnesium hydroxide, works as a flame retardant chemical for wire and cable applications. It also is the active ingredient in “Milk of Magnesia”, which is an oral antacid for heartburn and digestive issues. Other mineral fillers with flame retardant effect may not have this same dual effect and further, may not be safe for ingestion at all. Environmental chemical effects, as well as chemical persistence, bioaccumulation, and toxicity (PBT) profiles are very chemical structure dependent when interacting with humans and the natural environment. All mineral fillers may be persistent (and it is debatable that persistence for minerals is really a problem or not), but they will have very different bioaccumulation and toxicity profiles dependent upon their chemical structure. Likewise, all chemicals and chemical flame retardants will have different PBT profiles, even if they are in the same general chemical class. With this in mind, we can discuss some regulatory history of halogenated vs. non-halogenated flame retardant chemicals.

Halogenated flame retardants began use in earnest in the 1930s and onwards, as they were found to be potent flame retardant additives for flammable materials, as well as strong extinguishing agents such that liquid halogenated solvents were used in fire extinguishers. Indeed, there are reports of fire extinguishing “hand grenades” that were glass globes filled with carbon tetrachloride (now known to be a potent carcinogen) that firemen would lob into fires to help put them out, and, this same halogenated chemical was used in hand-held fire extinguishers [1]. As hazards of these liquid chemicals were found, these liquid halogenated flame retardants were pulled from service and other active extinguishing agents were instead put into fire extinguishers. Halon gas extinguishers were used for severe fire situations, but even these have been pulled from service due to ozone depletion issues. Their relative chemical stability made them non-toxic and therefore a preferred choice, however, for the same reason the chemicals were able to reach the stratosphere where they finally reacted with ozone. Halogenated flame retardant additives put into plastics began to be under regulatory scrutiny in the late 1990s to early 2000s as part of a move to prevent dioxin formation when end-of-life plastics (and other household waste) would be sent to incinerators. Incineration of waste is commonly carried out in Europe due to the lack of landfill space there, and for waste-to-energy efforts that are present in some European countries, especially in Scandinavia. Waste is difficult to presort, and so large amounts of polyvinyl chloride (PVC), as well as other halogenated compounds, ended up in the waste and large amounts of dioxin were formed as part of the emissions from these incineration facilities. As this was discovered, regulations were put in place to mitigate and cease dioxin formation via two methods. The first was with improved emissions capture and cleanup systems (baghouses, scrubbing systems, afterburners), and the second was to remove halogen from the waste stream. The second approach was where regulations against halogenated flame retardants began in earnest, with two well-known directives, the Reduction of Hazardous Substances (RoHS) [2, 3] and Waste Electrical and Electronic Equipment (WEEE) [4, 5]. These initiatives sought to reduce and eliminate the use of halogenated additives in consumer products, namely electronics, which would in turn reduce the amount of halogenated additives going to incinerators, or, accidentally released to the environment. The directives also aim at eliminating legacy brominated flame retardants from recycle streams, so that they do not end up in new E&E equipment via recycling.

Another reason for banning or limiting use of halogenated flame retardant additives in flammable materials (such as polymers) is the corrosive gases that form from these flame retardants as they activate in a fire. The vapor phase flame inhibition mechanism of halogenated flame retardants is well known to produce acid gases (HF, HCl, HBr) [6–9] which can present some secondary health effects (irritation of eyes and lungs) which can exacerbate the toxicity situation caused by the primary toxicant in fires, carbon monoxide [10–14]. Additionally, the acid gases can cause significant economic damage to materials that are sensitive to corrosive gases. Modern electronics are particularly sensitive to corrosive gas damage, and so there have been new regulations banning halogenated flame retardants from computer server facilities computer chip fabrication sites for this very reason. There are also some acidic gas regulations for aerospace, maritime, and mass transportation which also limit or effectively ban halogenated flame retardants from use.

Other European Union (EU) regulations have come into effect banning specific brominated flame retardant molecules found to have negative persistence, bioaccumulation, and toxicity (PBT) profiles, especially as new information comes to light indicating that a particular chemical structure is hazardous. This is how things evolve from a chemical use perspective, and is how it should occur. With new information about hazards, hazardous materials should be removed from use and commerce. However, as new information comes along, sometimes the regulatory picture becomes clouded. Going back to the main issue with dioxin formation, it is now well known that with halogen being naturally present everywhere in our environment, any time you have a fire or combustion event where halogen is present, you will form dioxins. Halogenated dioxins can be found in forest fires [15] as well as from electrical/electronic fires [16]. Unless you have capture systems and afterburners, dioxins will be emitted. The amount of dioxins formed depends on the materials involved in the fire event, as well as combustion conditions. It’s impossible to remove halogen from the environment, and indeed, fires themselves, especially accidental ones involving modern materials, produce all sorts of toxins and pollutants including sub-lethal gasses, lethal gases, and carcinogens such as polyaromatic hydrocarbons (PAHs) [17–22]. These toxins can be found in fires where flame retardants are present, as well as those without flame retardants, although the total volume of pollutants produced is less if the fire growth is lowered by the presence of effective flame retardants [23–29]. Therefore, the original regulatory reason behind halogenated flame retardant regulation and use (to prevent dioxin formation) is still correct, but with new information, the benefits and drawbacks of said regulations are now not as clear as they once were.

Stepping aside from the emission issue of hazards from halogenated flame retardant in fire events, there is the non-fire “emission” of the halogenated flame retardant when it gets into the environment. Going back to the above mentioned PBT issue, any chemical will be of concern in the environment if it should be emitted, spilled or introduced outside of controlled situations and the chemical is persistent (lasts for a long time), bioaccumulates (enters and concentrates in living organisms), and is toxic. Halogenated flame retardants of old are by design persistent due to their chemical structure, and the fact that one wants the flame retardant to last for years inside the product. One does not want to buy something with a 20 year lifetime only to have the fire protection wear out in the first year. This persistence has found halogenated flame retardants in many different places in the environment [30–39], and it is rightfully troubling. Many of the older halogenated flame retardants are small lipophilic molecules, meaning they can also be bioaccumalative (in the fatty tissue of many organisms), and some have also been found to be toxic. These negative PBT issues are why polybrominated diphenyl ethers (PBDEs), which are small molecule halogenated flame retardants, have been banned from use in the EU and US, as well as many other countries [3, 5, 33–38] By extension, several countries and US states have started to extend the bans on PBDEs to all halogenated flame retardants, regardless of chemical structure. It is important here to note that small molecule flame retardants are of concern when they migrate out of the plastic, but polymeric brominated flame retardants are of high molecular weight and while they are persistent, current data indicates they are not bioaccumulative or toxic. Likewise, reactive flame retardants which covalently bond into a polymer structure cannot get into the environment and cannot become bioaccumulative or toxic, even if they may be persistent. So wholesale bans on entire classes of chemicals may not be merited, but regardless of the lack of scientific merit, these wholesale bans are being implemented. Further, the volume of data against small molecule halogenated flame retardants having negative PBT profiles is such that even when halogenated flame retardants are polymeric or reactive, market conditions shy away from their use. Still, technology moves forward, as do opinions and personal/market tastes, and so there is still a need for fire safety protection/flame retardant chemistry, and therefore the market moves to non-halogenated flame retardants. Hence the reason for this book to guide materials scientists toward how to use non-halogenated flame retardant chemicals to provide fire safety, and to guide them on the newest information available.