Expandable graphite is the flame retardant of choice for the railway industry and our equipment makes sure this material reaches its full potential.
November 25, 2021
A wide variety of flame retardants
There are more than 175 different kinds of flame retardants on the market and are commonly classified into three groups, each containing subcategories:
- Halogenated flame retardants are the most popular because they are liquid and easy to mix. They guarantee excellent performance but are currently banned in the EU for their composition, and are under scrutiny in other countries due to the carcinogenic potential of the gasses they emit
- Organophosphorus and nitrogen-based flame retardants can be reactive or additives, have been used in large quantities in various consumer and building-related products for decades, but are harmful to human health
- Inorganic and mineral flame retardants guarantee the best performance in terms of smoke emission
Melamine belongs to this latter group, but is currently being phased out due to suspected harm to health. Expandable graphite, which received a significant boost in recent years also belongs to this group; this is apparent in the public transport sector, where the passenger safety is the primary concern.
Why expandable graphite?
Expandable graphite (EG) is of mineral origin; derived from carbon (which does not burn), it is composed 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 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 adding 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, norm EN 45545 is often cited as the reference regulation worldwide. Released in 2013 and updated this year, this norm prescribes testing the seat pad in a vandalized condition, with the fabric wholly torn and polyurethane foam directly exposed to a 15-kW power flame.
EG’s limitations
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 adding expandable graphite to seat padding would increase its weight too much.
Using expandable graphite inside polyurethane requires attention and expertise to prevent the graphite flakes from breaking and releasing the harmful sulfuric acid contained within.
In high-pressure polyurethane dosing units, expandable graphite undergoes mechanical stresses that may cause it to break. This happens mostly during high-pressure recycling at the mixing head when polyol when expandable graphite goes through the injector.
The sulfuric acid released generates a corrosion risk to some parts of the machinery. But more often it neutralizes the catalyst dissolved into polyol, affecting the foam’s final quality and must be minimized as much as possible.
Our solution
Our EG-AX system allows you to process expandable graphite without affecting the integrity of its flakes. It includes a multi-component mixing head, available in different sizes, provided with special injectors which reduce mechanical stress on the expandable graphite flakes.
This kit shortens high-pressure recycling times down to one minute or less.
When using this system, the formulation is kept intact much longer in the foaming machine, up to 3 times more than in “conventional” equipment.
