The effect of ageing on fire safety of composite materials

Products and materials are constantly developed and used in new applications and they are assessed regarding function, material properties and fire performance, before appearing on the market. The products and materials must fulfil national and international fire requirements and regulations to achieve a fire classification before the introduction to the market. The fire behaviour is assessed only on newly produced materials, as stated by the fire classification regulations. However, it is known that as material age, with time and environmental exposure, the material performance change. Studies show that mechanical properties, such as strength, are deteriorated when the material age. In literature, studies of the effects on aged materials regarding fire performance are limited, especially when dealing with composite materials.
Composite materials which commonly consist of a fibre reinforced polymer resin laminate and a combustible core material, are lightweight and show good mechanical properties. Therefore, they are used in many applications such as construction products, in marine and railway vehicle applications as well as in automotive. Despite their good mechanical properties, they can ignite and burn, hence the stability of constructions cab be compromised. Thus, it is of importance to have knowledge of the materials fire behaviour, their end use application, and how the material properties change when exposed to harsh environments and the change in properties with time as they age.
The objective of the PhD study was to evaluate the effect of ageing on the reaction-to-fire properties from accelerated ageing tests with six composite materials intended for applications on ships. Three composite laminates and three sandwich panels with intumescent coating was evaluated. The materials were aged using two accelerated ageing methods: thermal ageing with a temperature of 90 °C and moisture ageing with a moderately increased temperature of 40 °C and a relative humidity in the atmosphere of 90 %. Samples were collected from the ageing chambers after one, two and four weeks of ageing. The reaction-to-fire properties were evaluated using the ISO 5660-1 Cone Calorimeter and the EN ISO 5659-2 Smoke Chamber with FTIR (Fourier Transform Infrared) gas analysis. Parameters such as time to ignition, heat release rate and smoke production were evaluated with the Cone Calorimeter. With the Smoke Chamber, the smoke density, i.e. the visibility through the smoke was assessed, as well as the toxicity of the smoke.
The results from the ageing study evaluated with fire testing, showed that the fire behaviour of the selected composite materials in many cases was clearly affected by the accelerated ageing, both by the thermal exposure and by the moisture exposure. The effect was, however, inconclusive as the results showed both improved and deteriorated fire behaviour. One example showing the consistent behaviour is the basalt fibre reinforced phenolic composite laminate FRP 2. The thermal aged samples generally showed a deteriorated fire performance with a higher peak heat release rate, higher smoke production and higher concentration of toxic gas species in the smoke. On the contrary, the moisture aged samples showed improved fire performance with lower peak heat release rate and lower smoke density. However, the gas specie concentration in the smoke was higher for the aged samples.
Another example is one of the sandwich panels with intumescent coating, Sandwich 2. After thermal ageing, the fire performance was improved with lower peak heat release rate, lower smoke production, lower smoke density and lower gas specie concentration. The moisture aged samples showed a higher smoke production, higher smoke density and generally higher gas species concentration, thus the general effect from ageing was deteriorated fire performance.
However, all the fire test results for all six composite materials, have in common that the parameters were affected by the accelerated ageing. They were either improved or deteriorated, as there was a change from the unaged samples.
This study was limited to include tests with six selected composite materials. The results show that further research is needed within the topic, of ageing and the impact on materials fire behaviour. As the results were not consistent for the different materials investigated, further research and additional test data is required to give a deeper knowledge of the ageing mechanism and the effects from ageing.