25 Nov 2021

In specific contexts, such as the production of buildings, transport, or cars, flame retardants play a crucial role. They slow down combustion, giving people enough time to escape and survive. Besides preventing the spread of fire, flame retardants added to polyurethane formulations keep gases generated by combustion under control.

A wide variety of flame retardants

There are more than 175 different kinds of flame retardants on the market. They are commonly classified into three groups, each of which contains subcategories:

  • Halogenated flame retardants are the most popular because they are liquid and easier to mix. They guarantee excellent performance but are currently banned in the EU for some included materials, and are under scrutiny for others due to the carcinogenic potential of the gasses they emit;
  • Organophosphorus and nitrogen-based flame retardants can be either reactive or additive and have been used in large quantities in various consumer and building-related products for decades. However, they are dangerous to health;
  • Inorganic and mineral flame retardants: they guarantee the best performance in terms of smoke emission.

To this latter group belongs melamine, currently under investigation because of suspected harm to health, and expandable graphite, which received a significant boost in recent years. This has been especially apparent in the public transport sector where the safety of passengers is a big concern.

Why expandable graphite?

Expandable graphite (EG) has a mineral origin; it derives from carbon (which does not burn), and it is composed of packages of nanometric layers held together by sulfuric acid.

Its composition allows this material, once exposed to heat, to expand and create an intumescent layer on the material’s surface. Under the effect of heat, it increases its volume by about 300 times, forming a low-density worm-like structure on the surface of PU foam that prevents heat and oxygen transfer, slowing the spread of fire and minimizing the creation of toxic gases and fumes.

Expandable graphite’s applications

Polyurethane foams are inherently flammable, especially those with open cells. For this reason, some specific PU products require the addition of expandable graphite as a flame retardant.

In the public transport sector, in line with passenger safety, EG is employed to avoid the quick spread of fire, giving passengers time to escape. While in automotive products, it is used in both seating and acoustic insulation.

Though for the previous applications there are not yet any national or general regulations, for railways the norm EN 45545 seems to remain and be the reference regulation worldwide. Released in 2013 and updated this year, this norm prescribes testing the seat pad in a ruined condition, with the fabric wholly torn and polyurethane foam directly exposed to a 15-kW power flame.

Its limitations

However, expandable graphite as a flame retardant is not preferred for all polyurethane products: for example, it is not the best choice for furniture because of the black color it gives the foam. On the other hand, the aerospace sector often opts for other solutions for its seats since expandable graphite inside the seat padding would increase its weight too much.

The use of expandable graphite inside polyurethane requires attention and expertise to prevent the graphite flakes from breaking and releasing the sulfuric acid contained within.

In the high-pressure polyurethane foaming machine, expandable graphite undergoes thorough mechanical stresses that may crumble its small chips. This happens mostly during high-pressure recycling at the mixing head when polyol with expandable graphite goes through the injector.

The sulfuric acid released generates the risk of corrosion of some parts of the machinery, but it primarily neutralizes the catalyst dissolved into polyol, affecting the final quality of the foam. Thus, the grinding and crumbling of expandable graphite in a high-pressure foaming machine must be minimized or avoided completely.

The Cannon solution

The Cannon EG-AX system allows the processing of expandable graphite without affecting the integrity of the flakes. It consists of a multi-component mixing head, available in different sizes, provided with special injectors which reduce the mechanical stress on the expandable graphite flakes.

The EG-AX system shortens the high-pressure recycling time down to one minute or less.

When using this system, the formulation is preserved for much longer in the foaming machine, up to 3 times more than in “conventional” foaming machines.