PU Coatings Can Decompose By Phosphorus-Containing Flame Retardants

With the help of chemical instrumental methods, a deep insight into the interaction of different degradation processes of flame retardants and polymer coatings can be gained. Under climatically unfavourable conditions, a long-term stability of the functional coating can only be guaranteed to a limited extent, as these techniques clearly show.

The use of several phosphorus-containing flame retardants in polyurethane-coated textiles is considered to be critical due to the oxidation or hydrolysis sensitivity, write Sebastian Eibl and Matthias Müller of the ‘Wehrwissenschaftliches Institut für Werk- und Betriebsstoffe’ in the technical journal “Technical Textiles”.

Polyurethanes (PU) are very widely used in the textile industry, for example in adhesives, elastic threads and foams. In order to reduce flammability in textiles, halogenated flame retardants in combination with antimony trioxide were used in the past. Possible toxicological and ecological effects led to the development of phosphorus-containing alternatives. In addition, several flame retardants have often been combined to optimize the flame retardant properties. This proves to be negative in the durability of PU coatings.

More Damage Than Benefits For Coatings?

Various materials tested suffered damage to the PU coating, with colour changes, stickiness and odour development as well as corrosion with metallic applications.

Damage to the PU coating can be detected, for example, as a result of an oxidation and hydrolysis process during storage of functional fabrics in a folded state. This results in a conversion of phosphorus-containing flame retardants to o-phosphoric acid.

Areas of folded textiles which have contact with (outside) air show lower pH values, or a higher o-phosphoric acid content. With highly damaged coatings, a conversion of up to 46 per cent of the existing flame retardants into o-phosphoric acid and all that with a pH of 2.0 can be observed. Due to the instability of the polyurethane to strong acids the stickiness occurring can be attributed to a degradation reaction of the polyurethane, according to the authors.

Overall it can be considered to be a concerted degradation process in which the hydrolysis and oxidation of the flame retardants, or the degradation of the polyurethane polymer accelerate each other. In the long term a complete functionality of the textile coating can not sufficiently be guaranteed with regards to the required flame protection.