Fire-safe compounds: Standard raises burning contradictions

Based on article originally published in ERJ Jan/Feb 2016 issue.

The pan-European railways standard EN 45545-2 covers a wide range of fire safety measures, including materials and their hazard levels.

European standard EN45545-2 came into force in 2013 and will replace all national standards as of March 2016 across the EU; and as far as rubber parts are concerned, fire behaviour of the materials – known as FIRST (flammability, ignitability, rate of heat release, smoke, toxicity) – is the criteria to be met.

Concerns, however, have been voiced with regards to some of the requirements of EN45545-2, as consequences of emissions during and after a fire become equally as important as flame retardancy in the new standard.

“Our customers are concerned that these new standards show partially opposed requirements,” said Jorg Stumbaum, technical marketing manager, high performance elastomers business line CR/EVM at Lanxess.

A rubber part, he explained, has to have good flame retardancy, good high-temperature resistance (for long service time), good low temperature flexibility (to be used in colder regions of Europe), good physical properties (high tensile strength, low abrasion), sometimes good oil resistance and of course it must be cheap.

The difficulty, Stumbaum added, is to find the best compromise between all requirements.

It is not one size fits all, as Hubert Butterwegge, head of marketing and sales EMEA/Asia/Pacific at ContiTech Industrial Fluid Systems put it.

“We experience time and again that customers are searching for products that fulfil – in all their component parts – the European fire safety standard EN 45545. But it is quite difficult to meet all criteria because some flame retardants release toxins, for example,” he said.

According to Butterwegge rubber products need to fulfil their actual functions in addition to the fire standards. And for the “actual functions” it is important to meet other standards too.

For example, explains Butterwegge, hydraulic standards and the requirements from the International Union of Railways (UIC) are often a particular difficulty.

Another point of frustration for ContiTech is that customers do not appreciate problems with conflicting requirements.

“That is to say, they still don’t see a conflict with competing parameters like low-temperature flexibility, predetermined dimensions and elongation properties from the material; and the EN 45545-2,” he explained.

Another issue with the new standard is its stringent testing procedure. In the future, materials and components will have to meet the tests for flame propagation to ISO 5658-2, heat release in the cone calorimeter to ISO 5660-1 and for smoke development and toxicity in the smoke chamber to ISO 5659-2.

“The transition from national DIN 5510 to EN 45545, and, therefore, the test conditions and test methods, have changed strongly in some areas,” said Butterwegge of ContiTech.

To meet the new requirements, new material development is necessary for many articles.

For example, he said, to adapt our railway hoses to fire protection standard EN 45545, the company has to use synergies from the Continental Corp. and build on the expertise of other business units.

Stumbaum also pointed out that a big challenge in materials development was the rate of heat release during the fire test for halogen-free rubber parts, especially for high-hazard level classes in EN 45545.

Commenting on new flame-retardant materials technologies that could help meet the new requirements, Stumbaum, pointed to a combination of halogen-free elastomers with flame retardant fillers such as alumina trihydrate (ATH) or magnesium hydroxide (MDH).

ATH and MDH serve to retard both flames and smoke via endothermic reaction that releases water when heated to decomposition. The metal oxide by-products from the decomposition also help form a char on the polymer which insulates the polymer from heat and oxygen.

Clever compounding, explained Stumbaum, is the key to reduce costs and maximise effectiveness here.

“Many flame retardant raw materials are quite expensive and the product developers have to make the best combination of them in their compound to fulfil the requirements of the new standards and to be still affordable for the end user,” he noted.