Compounding the issue

Eric Moussiaux, Vice President Technology at Exel Composites, explains how fire considerations influence composite design.

Composites like fibreglass and carbon fibre can be used in many different building applications, such as window profile frames. They can provide more durable solutions, thereby helping to lower lifetime operational costs through reduced maintenance. The mechanical properties of composites – such as their weight, stiffness and strength – are normally the deciding factor in the design specification process.

However, in applications where there is a risk of encountering fire, this becomes the primary consideration – especially when Building Regulations state that for residential buildings over 18m in height, all materials on the external wall should be fire Class A1 or A2.

With the use of composites in demanding applications increasing, improving knowledge of their fire performance is a safety-critical issue. For this reason, Exel Composites has created a white paper discussing the practicalities for industries such as transportation and building, construction and infrastructure.

Generally, there are three main fire performance considerations for manufacturers. These are:

1. how difficult is it for the material to ignite, how fast does the fire spread, and how much heat does it generate?
2. how much smoke will develop and how quickly?; and
3. how toxic is the smoke, and how quickly can it cause harm?

These are referred to as the fire, smoke and toxicity (FCT) properties. It is worth noting that fire standards across Europe weren’t unified until 2000 with the Euroclass system, EN 13501-1. This system classifies the reaction to fire of construction and building materials based on three main factors: combustibility, smoke production and the production of flaming droplets or particles without focus on toxicity.

Toxicity

With these considerations in mind, it is common for composites manufacturers to incorporate flame retardant additives into the materials, reducing or delaying the combustion process. These are based on the use of non-halogenated substances such as aluminium trihydrate (ATH) and metal hydroxides, including magnesium and phosphorus-based compounds.

ATH additives are now the most widely used non-halogenated flame retardants because, despite being less effective than halogens, they are far less toxic when they burn. They operate by releasing water molecules in an endothermic reaction during the combustion process. These water molecules quench the surface of the surrounding materials, providing flame retardancy and smoke suppression.

The downside of ATH, other than being much less effective in hampering the fire, is that higher quantities of it are needed to meet the increasingly stringent fire requirements in many applications. Very high filler loadings negatively affect performance because the associated filler material leaves less room for reinforcing fibres, which can affect structural and mechanical performance.

As well as selecting appropriate FST tests, specifying engineers must ensure that the composites they choose are not hazardous to health. The white paper explains how a balance can be struck between the desired mechanical properties and meeting the relevant fire safety tests.

How to manage fire, smoke and toxicity requirements when specifying composites white paper bit.ly/Exel_whitepaper

Image credit | Exel-Composites