In this third article about thermoplastics Flame Retardant (FR) trends, we will discuss unmet flame retardant needs. Like any other needs, opportunities for new flame retardants should be validated against commercial interests and regulatory requirements before resources are spent on meeting them. With that caveat in mind, the discussion below reflects what this author believes to be some of the key problems to be resolved in the near future. The article describes thermoplastic uses that require either unusual processing conditions or applications that result in the need for significant changes in the performance of flame retardant materials. The resulting changes in design may greatly shake up the thermoplastic flame retardant material market.
Flame Retardants for Additive Manufacturing of Plastics
In the past year, a brand new technology has opened up new markets, novel applications, and the potential to bring plastic goods manufacture to a much wider range of companies. The technology causing this explosive change is known as additive manufacturing or 3D printing1.
Additive manufacturing has the potential to make plastic custom-made parts, designs, and shapes readily available similar in many ways to how the printing press made the written word much more available. While 3D printing technology has been around for many years, it has really taken off in the past year as can be seen from the recent coverage in the Economist2,3, and the recent announcement that Staples will print parts for the customers at their stores4.
So how does flame retardancy fit into all of this? The answer can be found by examining the application of the printed parts. More specifically by examining the use of the printed parts and any fire risk associated with those applications. If you are just printing a toy or a neat shape that will be used as a model, then any plastic compatible with the 3D printing process is acceptable. But if you’re making a custom case for an electronic application or a custom part then there may have a fire risk associated with it. If there is a chance of an electrical short circuit near a plastic part, flame retardancy may be required, and this need may not be met with common thermoplastics used for 3D printing (typically HIPS or ABS).
One approach that might be considered is printing with low flammability plastics (such as thermoplastic polyetherimide) but to date, there have been very few flame retardant materials designed and optimized for 3D printing.
With the probable explosion of plastic use in additive manufacturing processes, this need is clearly an unmet one, and could become a problem if additive manufacturing starts producing parts and goods inserted into applications without fire safety needs. Therefore, it makes sense that flame retardant thermoplastics optimized and tailored for 3D printing processes should be studied sooner rather than later. Very likely this will be met via small custom formulations at first, but this could be a growth area for flame retardant plastics.
The fire safety standard that regulates plastics for automotive applications is Federal Motor Vehicle Safety Standard # 302 (FMVSS 302) This is a simple horizontal burn flame spread test. that was created in the 1970s to simulate cigarette ignition and has not been significantly changed since its creation. At this time a typical car would have a maximum of 30 kg of plastic throughout the entire vehicle, all of which would have had to pass this simple test. The use of plastic to mitigate corrosion/rust damage and improve fuel efficiency (light-weight materials – better miles per gallon performance) has resulted in an increase in the amount of plastics in today’s cars to 150 kg or more. This increased level of plastic can significant add to fire risk. A post-crash fire is just one accidental ignition situation in which the ignition source may well be more intense than that provided by a cigarette, and any plastic needs to be flame-retarded to a level that allows enough time to escape from the burning vehicle5.
Despite several calls to change the automotive fire standard over the past few years, there have been only minor changes. So automotive plastic fire safety is an unmet need, but until a new fire safety standard is decided upon, it is hard to know what type of flame retardant performance will be required. Today, most plastics with no additional flame retardant at all will pass the FVMSS 302 test. The test of tomorrow will be based upon a fire risk scenario which is to be determined. If we look at the fire losses for automobiles today, it appears that there may not be an immediate problem (see figures below), but the situation requires vigilance in case the fire losses begin to increase in the near future.
Figure 3-1: Car loss fire statistics from NFPA6
This situation is further complicated because automobile fire risks are expected to change with new propulsion technology. The fire risk from a tank of gasoline igniting post-crash is very different than the fire risk associated with an electric car, in which electrical short circuit or battery explosion may be the issue. The risk of battery explosion from lithium ion batteries, however, may be resolved if lithium-air batteries are commercialised in the coming decade. Until then, lithium ion batteries must be designed with care to avoid any possible fire hazards.
In January of 2013, a fire occurred on board a Boeing 787s equipped with lithium ion batteries. While the exact causes have not been positively identified yet, extensive fire testing and safety testing was needed before the planes were allowed to fly again7. Depending upon the size and energy density of the batteries, this fire risk scenario could show up in more often as this technology is used in more applications,. Or, as these batters are used more widely, the engineering gets better at mitigating the fire risk introduced by this battery technology, Alternatively the risk could be removed completely if lithium-air replaces lithium-ion technology.
Cars powered with natural gas or hydrogen have more of an explosion hazard than fire hazard, which is different from fuel cells that introduce both flammable fuel and explosion risks. All of this points to a significant unmet need for fire safe materials in automobiles. Until the fire risks can be identified and regulators can design relevant tests, it will be hard to see how this need will be met in the future.
Flame Retardants Compatible with Recycling & Waste-To-Energy Processes
The final need we will look at in this article is the development of flame retardant plastics compatible with recycling and waste-to-energy processes.
In North America plastics are either reground, recycled or sent to landfill at the end of life of a product. In Europe, waste that cannot be recycled is incinerated with other municipal solid waste because of a the lack of landfill space. Incineration is a well-established process established in the 1970s. It was here that dioxins were identified from plastic waste and this led to the scrutiny of brominated diphenyl ethers that has dominated flame retardant/environmental science over the past four decades. Many European countries now have clean incinerator technology that can capture and destroy all dioxins before they get into the environment, so the problem has been solved due to deselection of some flame retardants and adoption of new incineration technology.
Even in North America, putting plastic waste into landfill is not a sustainable solution. In some cases, the plastic is of more value if it is recycled rather than if it was burnt in an incinerator for possible energy capture.
This means that there is an unmet need for plastics which can be easily recycled in addition to having value in waste-to-energy processes such as incineration and gasification. For flame retardant thermoplastics, the unmet need is for flame retardant additives compatible with the recycling process and/or energy recovery methods. Some flame retardant products are more compatible with recycling than others. Halogenated flame retardant chemistry has shown itself to work well with recycling (provided the additives do not bloom out over time)8 while some phosphate based flame retardants are not as suitable for recycling, owing to hydrolysis of the organophosphate over time9. There has been little work to study how waste plastics (including those with flame retardants) behave in waste-to-energy processes10 so there is still much to study. There is truly a strong need to give more attention to sustainability of thermoplastics in the coming decade. This should be accomplished through lifecycle analysis in which flame retardancy is considered as part of the “cradle-to-grave” design.
Flame retardant material design, and indeed fire safety in general, is a field of science, that is advanced as the result of reaction to problems. In general, flame retardants are developed in response to new or existing fire risks. The needs listed in this article are a combination of proactive (flame retardants for additive manufacturing) and reactive (automotive, flame retardants designed for sustainable plastic use) and may not be important if a bigger fire risk is discovered in the near future. Proactive flame retardancy of plastics is really only possible if one considers material flammability at the start of product design and extensive market and application research is carried out early in the project. Even then, one cannot predict the future, which depends on how consumers may use or abuse products, nor how technology may be applied in ways that create unexpected fire risk scenarios. So at best the scientist can look to literature reports, conference presentations, and trends seen by experts to help them plan for the future. They should accept that even with this information, they will need to vigilant and react to fire events, as these will have an important impact on the actions that they will need to take. It is hoped that this three part guide on flame retardants for plastics will be helpful to material scientists so they can be aware of the trends in flame retardant technology from a regulatory perspective (which drives flame retardant use), and from a technological perspective (what new technologies may solve today’s and tomorrow’s problems).
Again, readers are encouraged to read more on flame retardant technology to put the trends outlined in these articles into perspective for their needs, as there is no universal solution to flame retardancy of plastics. What may affect one group of polymers in one market may have no effect on that same group of polymers in another market. Still, the trends outlined in these articles do reflect current factors affecting flame retardant technology and should serve as a good starting point for the material scientist tasked with developing flame retardant materials.
1. A summary of this technology and its capabilities can be found in Wikipedia at http://en.wikipedia.org/wiki/Additive_manufacturing (accessed 1 Apr, 2013).
2. http://www.economist.com/blogs/schumpeter/2012/11/additive-manufacturing (accessed 1 Apr, 2013)
3. http://www.economist.com/node/21552901 (accessed 1 Apr, 2013)
4. http://www.cnn.com/2012/11/30/tech/innovation/staples-3-d-printing/index.html (accessed 12 Apr, 2013).
5. “Human survivability in motor vehicle fires” Digges, K. H.; Gann, R. G.; Grayson, S. J.; Hirschler, M. M.; Lyon, R. E.; Purser, D. A.; Quintiere, J. G.; Stephenson, R. R.; Tewarson, A. Fire and Materials 2008, 32, 249-258.
6. http://www.nfpa.org/itemDetail.asp?categoryID=953&itemID=29658&URL=Research/Fire%20statistics/The%20U.S.%20fire%20problem&cookie_test=1 (accessed 12 Apr, 2013).
7. http://en.wikipedia.org/wiki/Boeing_787_Dreamliner_battery_problems (accessed 28 April 2013)
8. (a) “Processing and properties of engineering plastics recycled from waste electrical and electronic equipment (WEEE)” Tarantili, P.A.; Mitsakaki, A.N.; Petoussi, M.A. Polym. Degrad. Stab. 2010, 95, 405 – 410 (b) “Waste electrical and electronic equipment plastics with brominated flame retardants – from legislation to separate treatment – thermal processes” Tange, L.; Drohmann, D. Polym. Degrad. Stab. 2005, 88, 35-40,
9. “Artificial Weathering and Recycling Effect on Intumescent Polypropylene-based Blends” Almeras, X.; Le Bras, M.; Hornsby, P.; Bourbigot, S.; Marosi, Gy.; Anna, P.; Delobel, R. J. Fire Sci. 2004, 22, 143-161.
10. (a) “Controlled pyrolysis of polyethylene/polypropylene/polystyrene mixed plastics with high impact polystyrene containing flame retardant: Effect of decabromo diphenylethane” Bhaskar, T.; Hall, W. J.; Mitan, N. M. M.; Muto, A.; Williams, P. T.; Sakata, Y. Polym. Degrad. Stab. 2007, 92, 211-221. (b) "Kinetic analysis of thermal degradation of recycled polycarbonate/acrylonitrile-butadiene-styrene mixtures from waste electric and electronic equipment" Balart, R.; Sanchez, L.; Lopez, J.; Jimenez, A. Polym. Degrad. Stab. 2010, 91, 527-534. (c) “Chemical Recycling of Polymers from Waste Electric and Electronic Equipment” Achilias, D. S.; Antonakous, E. V.; Koustsokosta, E.; Lappas, A. A. J. App. Polym. Sci. 2009, 114, 212-221.