In the previous article, we discussed flame retardant (FR) regulations and how they might be expected to change. Nevertheless, the market is still strong for non-halogenated flame retardants. Therefore, the objective of this second part article is to summarize notable experimental results obtained with commercial Flame Retardants, and approaches that are likely to be important over the coming decade. New Flame Retardants chemistries and approaches will also be discussed.
New Commercial Non-Halogenated Flame Retardants
The novelty of the new non-halogenated flame retardants means that academia and government labs have not done a lot of research with the materials. The information in this section will be based solely upon the limited publically available information. Most of these new flame retardants are polymeric in structure and their chemical structures have not been published; although manufacturers have sufficient technology to provide general guidelines on how to apply them, it is possible that their structures have not yet been elucidated.
However, these materials would not be commercial at all if they didn’t show significant potential for flame retardant use.
The first new group of non-halogenated flame retardants is polymeric/oligomeric phosphonate compounds made by a company known as FRX Polymers. This company has also commercialised compounds which are copolymers of polycarbonate (poly(phosphonate-co-carbonate))1.
FRX Polymers has a range of polymeric additives and reactive oligomers, all of which can be melt compounded with other polymers, or can be used directly as inherently flame retardant materials. Applications in fibers, electronics, transportation (aerospace, trains), building and construction have been claimed. The polymers which appear to be most effective are those containing oxygen in the polymer backbone, including polycarbonate, polyesters [unsaturated, poly(ethylene terephthalate), poly(butylene terephthalate)], thermoplastic polyurethanes, and epoxies. Little is known about the properties of these flame retardant materials except for the minimal fire safety values provided by LOI (limiting oxygen index) test data. LOI test data does not correlate to any realistic fire scenario2, however, any material with a LOI of 60% or greater is a low heat release / low flammability material with potential as a flame retardant additive. Because of this, these promising flame retardant materials have been commercialized.
Israel Chemicals Limited have recently released Fyrol HF-5, another polymeric non-halogenated flame retardant. Although little is yet known about the material, it contains a high level of phosphorus, which suggested that it could be effective in flexible polyurethane foams used in automotive, mattress, and furniture applications3.
The final new non-halogenated flame retardant of note is FP-2100J, produced by Amfine Chemical Corporation. This material is an intumescent flame retardant additive package optimized for polyolefins. It is not polymeric, but shows efficient intumescent behavior by forming protective chars with very high thermal durability. About 20-25wt% loading of FP-2100J in polypropylene provides UL-94 V-0 performance at 1.6 and 0.8mm. Since polypropylene is flammable (one of the highest heat release polymers known), this level of performance is quite impressive for a non-halogenated material4.
New Experimental Non-Halogenated Flame Retardants
Outside the commercial arena, there have been two experimental non-halogenated flame retardants that deserve attention. They are boronic acids for polyurethanes, and deoxybenzoin co-monomers. It should be noted that of the new flame retardants reported, these two are truly of new chemistries. There have been other chemistries discussed and published in the past year, but they either build upon well-known chemistry (phosphorus, mineral fillers) or are still too new to science to discuss in detail.
In recent studies, boronic acids have been found to be effective as flame retardants for polyurethanes5. Boronic Acids have a similar structure to carboxylic acids (such as acetic or benzoic acid) but the central carbon has been replaced with boron. An example is provided by the chemical structure in Figure 2-1.
Figure 2-1: Monoboronic Acid
These compounds showed notable reductions in heat release, changes in thermal decomposition behavior (suggesting crosslinking and char formation in the polyurethane), and appeared to have slowed or preventing dripping and flow of the polyurethane during burning. In the figure below, the heat release of the pure thermoplastic polyurethane before and after addition of the boronic acid, flame retardant is shown.
Figure 2-1: HRR plot of TPU control (left), TPU + Monoboronic Acid (right); Below: Chars of neat TPU (no FR, left bottom), and TPU + Monobornic acid (right bottom)
Deoxybenzoin co-monomers (Figure 2-2) are notable in that they can copolymerize with a wide range of polymer chemistries, and they reduce heat release without the use of phosphorus or other typical non-halogen materials.
Figure 2-2: Bisphenol Deoxybenzoin Monomer
These materials show very little heat release when they thermally decompose to form a carbon char and water and then burn; therefore they serve mostly as a diluent. A 50/50 blend of deoxybenzoin and bisphenol A epoxy would show a 50% reduction in base heat release, while a 25/75 blend of deoxybenzoin and bisphenol. A epoxy would show a 25% reduction in base heat release. From the results published to date6,7,8,9this strategy appears to be a very promising path to lowering the base flammability of a commodity polymer without compromising mechanical properties or using other flame retardant additives. The chemistry is not yet commercial, but it seems simple to synthesize so that it could be rapidly commercialised in response to the right economic incentives.
Protective Coating - New Flame Retardant Approaches
Another way to achieve flame retardant protection in thermoplastic materials is through the use of protective coatings. There are two notable approaches relevant to thermoplastics. The first is the use of an infrared reflective coating on a thermoplastic so that the material never heats up enough to thermally decompose and ignite. For example Prof. Bernhard Schartel (of the German Federal Institute for Materials Research and Testing) placed a copper mirror on a thermoplastic material to dramatically delay the time to ignition of the thermoplastic material10. What makes this approach intriguing is that in the absence of flame retardants, significant ignition resistance can be achieved for a thermoplastic simply by reflecting the heat away from the plastic. While interference (EMI) shielding effects for the plastic with this copper mirror have not been measured, it seems likely that this mirror would be of great value for plastics in hand-held communication devices such as smart phones in which EMI and flame retardancy are crucial.
Another protective coating approach is provided by the use of layer-by-layer (LbL) coatings11,12,13,14. These have been used successfully for textiles and foams. With the LbL process, a polymer nanocomposite is built up one layer at a time on foam or textile materials, where it creates an unbroken (conformal) barrier over the polymer. When the material is exposed to a flame, the LbL coating chars and carbonizes to prevent the underlying material from degrading it further. This prevents dripping and/or structural collapse of the underlying material. The technology is a recent discovery and can be tailored to a wide range of polymer chemistries, but may only work well with high-surface area polymer constructions such as foams, and textiles. Flammability of nanocomposite coatings on cotton fabrics as a function of bilayers is shown in Figure 2-3. Some data however, suggests that the LbL coatings will not provide enough protection to a thick (>1mm) plastic material, but more data is needed to confirm this.
Figure 2-3: Mass loss and flammability of nanocomposites coatings on cotton fabrics as a function of bilayers (BL), (Li, Y-C; Mannen, S.; Yang, Y-H.; Morgan, A.B.; Grunlan, J.C., "Anti-Flammable Intumescent Nanocoatings on Fabric", Advanced Materials, 2011, 23, 3926-3931)
Polymeric Flame Retardants – Recent Trends
As mentioned briefly in part I of this series on flame retardant trends, many producers of flame retardant additives are pursuing polymeric materials. This is because these materials have a better environmental profile when compared to small molecules. All of the major flame retardant producers now have commercial polymeric additives, but the chemistry has not been widely revealed due to the proprietary nature of these additives. Recently commercialized polymeric flame retardant additives are described below.
The Albemarle Corporation15 provides two polymeric flame retardants, both of which are brominated. The first is GreenArmor, a polymeric flame retardant of unknown chemical structure. The chemistry could be based on brominated polystyrene since it contains a high level of bromine and is claimed to be optimal for HIPS and ABS.
The second compound is GreenCrest16, which is optimized to provide flame retardancy for polystyrene insulation foams, like those used in building insulation products. It is marketed as a replacement for hexabromocyclododecane (HBCD), but it is not clear whether this additive can replace HBCD in applications other than polystyrene foam, such as textile back-coatings.
Chemtura / Great Lakes Solutions markets and sells polymeric flame retardants under the“Emerald Innovation” tradename. The first is Emerald Innovation 100017, which shows good flame retardant performance in HIPS, ABS, and PP. It is also likely a brominated polystyrene, but since it shows some promise in PP as well, it may have some other polymer chemistry present which enables it to show good effectiveness in PP for simple ignition resistance tests like UL-94 V.
On the other hand, Emerald Innovation 300018 is optimized to provide flame retardant protection for polystyrene foams, and, like GreenCrest, it is a replacement for HBCD in polystyrene foams. This flame retardant is the material that Chemtura licensed from Dow Chemical a few years back, which suggests that it has been found to be very effective in polystyrene foams.
Israel Chemicals Limited (ICL) has a much wider range of polymeric flame retardants than the other companies19, with either halogenated and phosphorus-based chemistries. Its phosphorus based chemistries include Fyrol HF-5 and Fyrol PNX; both of these are optimized for polyurethanes. A wide range of brominated polymeric flame retardants has been listed on its website, covering brominated polyacrylates, brominated polystyrenes, and some proprietary chemistries as well. The full range of ICL’s offerings can be found at their website.
FRX Polymers has the range of polymeric additives and reactive oligomers that were described in the New Commercial Non-Halogenated Flame Retardants section of this article.
As outlined in Part I of this series, new flame retardants are being discovered, optimized, and commercialized. Polymeric flame retardants are dominating the new chemistries from the main commercial vendors (Albemarle, Great Lakes Solutions, ICL) and the new vendors (FRX). Flame retardants with very different chemistry (boronic acids for polyurethanes) and low flammability monomers (deoxybenzoin) are satisfying needs in a number of niches.
Coatings are being developed that provide fire protection through reflection of heat or through the charring of nanocomposite materials. Just as there are multiple ways to provide flame retardant properties to polymers and these must be customized to each application. The advances over the past year strongly suggest that there will continue be multiple solutions to choose from. Material scientists should expect existing flame retardant companies to continue to provide innovation, but they should also watch the literature for new small companies that can be expected to commercialize some of the approaches outlined in this paper.
1 http://www.frxpolymers.com/(accessed 13 Mar, 2013)
2 “Oxygen Index: Correlations to Other Fire Tests” Weil, E. D.; Hirschler, M. M.; Patel, N. G.; Said, M. M.; Shakir, S. Fire and Materials 1992, 16, 159-167.
3 http://icl-ip.com/?products=fyrol-hf-5(accessed 13 Mar, 2013)
4 http://www.amfine.com/productdetails.asp?ProductID=50(accessed 13 Mar, 2013)
5 “Synthesis and flame retardant testing of new boronated and phosphonated aromatic compounds” Benin, V.; Durganala, S.; Morgan, A. B. J. Mater. Chem. 2012. 22, 1180-1190.
6 “Flame Resistant Electrospun Polymer Nanofibers from Deoxybenzoin-based Polymers” Moon, S.; Ku, B-C.; Emrick, T.; Coughlin, B. E.; Farris, R. J. J. App. Polym. Sci. 2009, 111, 301-307.
7 "Halogen-free, low flammability polyurethanes derived from deoxybenzoin-based monomers" Ranganathan, T.; Cossette, P.; Emirck, T. J. Mater. Chem. 2010, 20, 3681-3687.
8 “Deoxybenzoin-Based Polyarylates as Halogen-Free Fire-Resistant Polymers” Ellzey, K. A.; Ranganathan, T.; Zilberman, J.; Coughlin, E. B.; Farris, R. J.; Emrick, T. Macromolecules 2006, 39, 3553-3558.
9 “Deoxybenzoin-based epoxy resins” Ryu, B-Y.; Moon, S.; Kosif, I.; Ranganathan, T.; Farris, R. J.; Emrick, T. Polymer 2009, 50, 767-774.
10 “Sub-micrometre coatings as an infrared mirror: A new route to flame retardancy” Schartel, B.; Beck, U.; Bahr, H.; Hertwig, A.; Knoll, U.; Weise, M. Fire Mater. 2012, 36, 671-677.
11 “Layer-by-layer assembly of silica-based flame retardant thin film on PET fabric” Carosio, F.; Laufer, G.; Alongi, J.; Camino, G.; Grunlan, J. C. Polym. Degrad. Stab. 2011, 96, 745-750.
12 “Development of layer-by-layer assembled carbon nanofiber-filled coatings to reduce polyurethane foam flammability” Kim, Y. S.; Davis, R.; Cain, A. A.; Grunlan, J. C. Polymer 2011, 52, 2847-2855.
13 “Clay-Chitosan Nanobrick Walls: Completely Renewable Gas Barrier and Flame-Retardant Nanocoatings” Laufer, G.; Kirkland, C.; Cain, A. A.; Grunlan, J. C. ACS Applied Materials & Interfaces 2012, 4, 1643-1649.
14 “Intumescent All-Polymer Multilayer Nanocoating Capable of Extinguishing Flame on Fabric” Li, Y-C.; Mannen, S.; Morgan, A. B.; Chang, S.; Yang, Y-H.; Condon, B.; Grunlan, J. C. Advanced Materials 2011, 23, 3926-3931.
19 http://icl-ip.com/?page_id=56 (accessed 20 Mar, 2013)
Alexander B. Morgan, Ph.D.
After receiving a B.Sc from the Virginia Military Institute (1994) and a Ph.D. from the University of South Carolina (1998), Dr. Morgan has worked for over seventeen years in the areas of materials flammability, polymeric material flame retardancy, fire science, fire testing, and fire safety engineering with an emphasis on chemical structure property relationships and fire safe material design. His current research areas include New Flame Retardant Technology for Polyurethane Foam and Furniture, New Flame Retardant Technology with Reduced Environmental Impact, Fire Testing Method Development, Waste-To-Energy Pyrolysis and Combustion Science and Thermal Degradation and Stability Behavior of Materials.
Dr. Morgan has helped academic, government, and industrial customers solve their flame retardant and fire safety needs in a wide range of applications. He is on the editorial review boards for two fire safety journals (Fire and Materials, Journal of Fire Science), and is a member of ASTM, Sigma Xi, International Association of Fire Safety Scientists, and the American Chemical Society.