Time is a life saver when discussing fire safety. All products designed to be flame retardant are intended to keep people safe by either preventing a fire or slowing one down - allowing time to escape danger. Flame retardancy is built into most everything around us - home furnishings, offices, vehicles, airlines, commercial marine and even some apparel.
For designers of flame retardant products, there is a myriad of things to consider before starting development work with materials.For example:
- Does a product need to be hard, soft or elastic?
- Does the appearance matter – glossy, matte, clear, or pigmented?Is a product visible in use or applied where the final consumer will not see it?
- Does a product need to be wash and water durable?
- What test methods will be used?
- Will a flame retarding system survive the fabrication process?
- What are the acceptable regulatory limits for materials?
Understanding the flame retardant test needed to pass is critical to designing products. The countries and states standards vary by application. For example:
- National Fire Protection Association (NFPA) standards, such as NFPA 701 Vertical Burn for Fabrics
- Underwriters Laboratory UL 94 for Plastic Flammability
- Motor Vehicle Safety Standard MVSS302
- ASTM E-.84 Tunnel Test
- British Standard 5852 for Upholstery
- Canadian Standards Association CSA
- California TB-603 Mattress Burn
- German DIN 4102 – B1
- French NFP 92503 M1
The list goes on. They all have a common goal of protecting the public and allowing time to escape a fire situation. And there are independent laboratories to test the final products around the world.
Assuming the developer understands the testing, what tools are available to build a flame retardant product? Fundamentally the approach to put out a flame is to take away one or more legs of the fire triangle: fuel, oxygen, and heat.
Fuel
In coatings and composite products, the fuel can be the substrate for example paper, cotton, polyester, nylon fabrics, films, wood, and combustible polymers used in the construction industry. Where possible, materials should be chosen that do not contribute to the fuel in a fire situation. Glass and ceramic fibers do not burn, and some highly-engineered materials are very resistant to combustion like Nomex®, Kevlar®, and carbon fibers.
One test of combustibility is the Limiting Oxygen Index Test (LOI%) ASTM D2863. This test measures the minimum amount of oxygen as a percentage need to support combustion of a polymer. The higher the number, the better the flame resistance. Atmospheric oxygen content is 21%. LOI above 21% is considered flame retarding and below 21% a polymer is considered fuel.
Limiting Oxygen Index (LOI) of Commonly Used Materials
| Material | % LOI | Onset of Decomposition °C / Melt Point (Tm °C) | Flame Retarding Component |
|---|---|---|---|
| Glass / Mineral Fibers | NA | NA | Does Not Ignite |
| PTFE | 95 | 300-327 - Does Not Ignite | Fluorine |
| PVDC & Copolymer | 60 | 225-232 | Chlorine |
| PVC & Copolymer | 37-39 | 245-270 | Chlorine |
| NOMEX® | 28.5-30 | 350 | Para-Aramid Fiber |
| NHFR Polymer (Lubrizol) | 29.5 | 175-390 | Nitrogen/Phosphorus |
| Kevlar® | 29 | 500 | Meta-Aramid Fiber |
| Wool | 24-25 | 500-600 - No Melt | Amide NOx |
| Brominated Polyurethane | 23 | 160-200 | Bromine |
| 21% Oxygen In Atmosphere | |||
| Polyester | 20-21 | 560 / Tm = 249 | Fuel |
| Nylon | 20-21.5 | 532 / Tm = 249 | Fuel |
| Polypropylene | 17-18 | 260 / Tm = 133 | Fuel |
| Acrylic Binder | 18-20 | 210 | Fuel |
| Cotton / Paper | 18-21 | 300 / Slow Decomposition | Fuel |
| Styrene-Butadiene | 18 | 150-350 | Fuel |
Heat and Oxygen
These other two legs are interrelated as far as the steps to remove them from a fire front. Many flame retardant chemistries work in the gas phase. The gas evolved from polymers or additives works to eliminate oxygen from the flame front. Common gas phase materials include nitrogen, halogens like bromine or chlorine, phosphorus compounds and water. At the same time, there is a cooling effect as these materials decompose. Another mechanism is charring. A char insulates the materials from the flame front and provides a barrier to combustion.
The decomposition temperature of flame retardant materials varies greatly. It is important to match the decomposition temperatures with the final composite in question. For example, a low melting fiber like polyethylene or polypropylene will melt at 130 °C before the temperature of decomposition. A composite made of glass fiber will not shrink from the flame, and a cellulosic material like paper or cotton will stay in the flame front longer and see more heat than a low melt synthetic. APP, ATH, MC, and DBDPE are commonly used.
Typical Decomposition Temperatures of Common Flame Retardant Materials
| Material | Onset of Decomposition °C | Active Ingredient or Flame Retardancy | Weight % of Gas Phase |
|---|---|---|---|
| Ammonium Carbonate* | 58 | Carbon Dioxide + Water | 67 |
| Aluminum Tri-Hydrate (ATH) | 230 | Water. Char | 35 |
| Ammonium Poly Phosphate (APP)* | 285-290 | Phosphorus Oxides. Nitrogen Oxides. Oxides | 73 |
| Melamine Cyanurate (MC) | 305-320 | Nitrogen & Oxides. Water. Char. | 49 |
| Magnesium Hydroxide | 330 | Water. Char | 31 |
| Decabromo Diphenylethane (DBDPE) | 330 | Bromine. Char | 82 |
| Calcium Carbonate | 825 | Carbon Dioxide. Water | 44 |
Flame Retarding Synergists
Common synergists, such as antimony trioxide, antimony pentoxide, and zinc borate, are included in flame retardant formulations in order to boost overall performance and economic cost. Antimony compounds are often combined with brominated and chlorinated flame retardants. Antimony Tri-Oxide is typically used with halogenated polymers and additives in a ratio of 3:1 to 5:1 halogen to Antimony Trioxide ratio. Antimony Pentoxide is a colloidal particle that is clear in appearance vs the antimony trioxide that is much larger and opaque coatings. Boron compounds function similarly with halogenated flame retardants and operate in the condensed and vapor phases of combustion.
Additional Considerations for Flame Retardants
Flaming drips can occur when the burning substrate breaks or drops from the sample and continues to burn. Many test methods require flaming drip to extinguish within a specified amount of time. Other test standards do not allow for any flaming drip. Though flaming drip can be difficult to overcome, particularly in certain substrates, additives such at melamine cyanurate can be added to flame retardant compounds. Higher degrees of crosslinking within the compound or the substrate can also aid in preventing the flow of the coating, which in turn, can help to reduce flaming drip.
While halogenated flame retardants are extremely effective in preventing a fire, the smoke and volatiles generated by the fully developed fire have come under scrutiny. Smoke density and smoke-toxicity is of immense concern when considering the survivability of a fire situation. The focus on halogen-free flame retardants has increased for smoke reduction as well as decreased smoke toxicity. Hydrated fillers have also proven to impart a powerful smoke suppressing effect in flame retardant coating.
In addition to flaming drips and smoke generation, afterglow is regularly measured and limited during flame testing. Afterglow is the residual smoldering combustion that remains after the flame is removed. It is a concern as it can possibly cause reignition of fire even after the initial flame is extinguished. Depending on the substrate, ammonium polyphosphate, zinc borate, and melamine are commonly used to suppress, reduce, or prevent afterglow.
Intumescent Coatings
A special flame retardant approach is to make coatings that form a foamy char in the presence of flame that insulates the surface from the flame. The char layer blocks both the heat and the oxygen at the flame front. The most common intumescent systems utilize the following components on a dry basis.
- An acid source (ammonium polyphosphate) [Phosphoric acid] - 3 parts
- Carbon source (di-pentaerythritol) - 1 part
- A blowing agent (melamine) [nitrogen blowing agent] - 1 part
- Binder to hold this together –[ideally char forming itself] - 1-2 parts
The weakness of the intumescent coatings are they are not very durable and can be damaged by water.
Ensuring the Right Protection
Flame retardant chemistries are generally halogen containing, phosphorous based, or intumescent and are combined fillers and synergists (or blends of said chemistries) to function in one or more phase during the mechanism of combustion. In order to create a successful flame retardant system, coating, or composite, many factors must be considered. The substrate to be treated (or coated) will dictate the type of flame retardant selected. The tests that the substrate of composite must pass will influence the levels of flame retardant. Finishing and coating method will help to define the physical properties of the FR system. Cost considerations and health/environmental constraints will shape the formulation process as well.
Lubrizol can help formulators identify a solid approach to exploring the right chemistries to consider for diverse flame retardant applications. Contact us to take the next step.
NOMEX® and Kevlar® are trademarks of DuPont de Nemours, Inc.
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