Fires in nightclubs in Spain have caused at least 162 deaths in the past 45 years. In the latest incident, recorded in Murcia, 13 young people lost their lives. Another tragic night occurred in 1983 at the Alcalá 20 nightclub in Madrid, where 81 people died.
Short circuits or improperly extinguished cigarettes can ignite a spark, causing nightclubs to blaze like torches with hundreds of people trapped in a ring of fire. Inside, everything, from curtains to sofas, is made of highly flammable polymers. The music and lights rely on electronic components, often the epicenter of the inferno.
The same holds true in our homes: spaces like bedrooms or kitchens contain everything necessary to fuel flames.
Surrounded by Plastics
Almost all materials used in interior decoration and furniture are highly flammable. This includes curtains, mainly made of polyester fabrics; sofa cushions (polyurethane foam); carpets (synthetic fibers); tables and chairs (wood and plastic); wallpaper (wood/plastic fiber), and more.
Electronic equipment such as TVs, computers, and sound systems also contain large amounts of flammable materials like thermoplastic polyurethane (TPU), ethylene-vinyl acetate (EVA), polyamide, epoxy resins, polyethylene (PE), and polypropylene (PP).
Why They Burn So Well
The vast majority of these materials are synthetic polymers, commonly known as plastics.
Their low manufacturing cost, high strength-to-weight ratio, versatility, and durability make them the preferred choice for various applications.
Most synthetic polymers consist of carbon, hydrogen, and oxygen atoms in long molecular chains. These longer chains generally result in stronger materials: the more building blocks used in their manufacture, the stronger the polymer.
The bad news is that they are highly volatile when exposed to high temperatures (above 250 °C), where these long chains lose their molecular stability. This volatility makes them highly flammable.
As the material is exposed to higher temperatures, the molecular chains within the polymer begin to degrade and break apart.
Free radicals, unstable structures where the bond with the general molecular chain has broken, start to form. Combustible gases are then released, which, in the presence of sufficient oxygen, initiate combustion.
As the material decomposes, the combustion process becomes increasingly aggressive; the material becomes more volatile, releasing more combustible gases until the fire sustains itself. If not controlled, it will quickly reach its most dangerous phase: flashover.
Flashover: When Escaping Might No Longer Be an Option
Once a fire is triggered, we have 3 to 5 minutes, or even less, before flashover is reached. This occurs when most materials in a room or enclosed space reach their autoignition temperature, the temperature at which they will spontaneously ignite without direct contact with an external ignition source. Flashover usually occurs at around 500 °C.
For example, in the recent fire at The Station nightclub in Rhode Island, sparks from ignited fireworks inside the venue set fire to highly flammable polyurethane foam covering the discotheque’s ceiling and walls, leading to flashover in less than a minute, resulting in the death of 100 people.
A Solution Without Replacing Materials
The omnipresence of these materials makes it impossible to eliminate them from our lives. However, we are working on a more promising and practical approach: making them less prone to ignition.
This is the focus of research by the High-Performance Polymers and Fire Retardants Group at the IMDEA Materials Institute.
The first line of research aims to neutralize flammable gases and reduce free radicals through a flame retardant additive. As the molecular structure of the polymer breaks down, releasing combustible gases, the additive releases non-combustible inert gases. These serve to dilute the concentration of oxygen and fuel in the flame zone.
We also investigate flame retardant additives that create a protective carbon layer as the polymer begins to burn. The formation of protective carbon layers, acting as a barrier to slow the flow of heat and mass, is likely the most significant mechanism in the condensed phase for flame retardation in polymers.
And it not only serves as a barrier for heat and mass flow but also as a means to preserve carbon, thus reducing its conversion into volatile flammable compounds.
A Universal Retardant
The challenge we face is that the polymers used to make a toy, curtain, or electronic cables have different flammability characteristics, so flame retardants cannot be generic.
For example, in recent research on thermoplastic polyurethane (TPU), a common component in electronic cable manufacturing, introducing flame retardant additives equivalent to 5% of the total material weight resulted in immediate extinction after removing the external ignition source.
However, for another common polymer in cable production, ethylene-vinyl acetate, we found that it is necessary to introduce additives equivalent to 55% of the source material weight. That is, 11 times more than required in TPU to achieve a similar result.
Another example is a flame retardant in epoxy resin developed at the IMDEA Materials Institute. By introducing 3% fewer flame retardants and nanomaterials, the maximum heat release rate is reduced by over 60%, and total smoke production by over 40%.
Why Aren’t These Retardants Used in the Industry Yet?
The first reason, unsurprisingly, is economic. The introduction of flame retardants in polymers can increase manufacturing costs. But the main obstacle is not the price; it affects the mechanical performance of the polymer.
At present, our main goals as fire safety researchers are to improve the performance of flame retardants and minimize the effect of these additives on the properties of the polymer itself.
We will continue researching to make our homes, offices, buildings, and nightclubs safer, reducing the risk of another tragedy until we make it preventable.