Non-halogenated Flame Retardant Handbook. Группа авторов
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.
1.2 Regulations of Fire Safety and Flame Retardant Chemicals
With some basic history about halogenated and non-halogenated flame retardants in place, we can now discuss more detailed regulation of flame retardants. In general, regulations are mostly reactive to information and events, rather than proactive to potential or perceived hazards. There are exceptions, but this reactive mode of regulation is applied in the majority of regulatory cases.
Modern fire safety regulations are often found within various legal codes, especially building codes, aviation regulations, and federal registers that describe particular requirements and test methods to ensure fire safety in a structure, vehicle, component, sub-component, or material. These regulations do not require any particular flame retardant chemistry to be used, but instead prescribe a particular level of performance. In fact, regulations really do not mandate flame retardants to be used at all. Flame retardants get used because it is one of many ways to provide fire protection, and may be selected depending upon all the other “non-fire” requirements for a functional item, including cost, thermal/mechanical/electrical performance, manufacturing requirements, intellectual property, and so on. It is important to emphasize this point as there is some perception that fire safety regulations mandate or push the use of flame retardants. This is not correct. The only time a particular chemical will be mandated for use is when it is prescribed in a manufacturer requirement document after certifications for use have been achieved. For example, a composite part inside the cabin of an aircraft that meets flame spread and heat release requirements and has been deemed “airworthy” may have manufacturer requirements to hold to a particular polymer formulation to ensure the part meets the requirements and does not have to be recertified for use. This requirement may then specify specific flame retardant chemicals, and loading levels, to meet the performance. But again, if one reads the original fire safety requirements, the original laws will not mandate any particular approach or chemical to be used.
Fire safety regulations will seek to mimic a particular fire risk scenario where there has been a notable hazard identified, and some probabilities of that hazard occurring with notable loss of life or property. Within the regulation is a test method that seeks to mimic the fire risk scenario,