Discoloration in Fire Retardant Polyurethane flexible foams

Discoloration of flexible PU foams also know as “scorching” is usually attributed to thermal degradation and oxidation and is particularly visible in some flame retardant foams. Four water blown fire retardant foams and one non fire retardant foam were studied. Fire retardants were applied at the levels of passing the California TB117 (A/D) test. When freshly prepared foam was subjected to the microwave heat yellow-brown discolouration appeared on the foams, which is dependent on the time of exposure and the presence of flame retardant. If the foam is aged in the air ventilated oven in humid air at 150°C, eventually all samples, including non flame retardant foam will develop color, with only difference that it happens faster in foams containing Fyrol FR-2 and Fyrol PNX. This experiments prove that scorching of PU foam is a kinetically controlled process and some flame retardants can accelerate it, probably without significant change of the mechanism. UV reflectance spectra of the scorched foam showed continuous increase of absorption in visible region at 400-700 nm and a new peak also appeared at 380 nm which, should not contribute to the color occurrence. Infrared of the scorched foam showed a progressive decrease of free isocyanate groups and modifications in the area of urea/urethane/NH2 groups absorptions. Isocyanate is known to be the most reactive functionality in the PU formulation. Hydrolysis of –NCO, which gives primary amines is the fastest reaction in the water blown foam. In the unaerobic conditions, the primary amines promptly react with free isocyanate forming urea linkages. However, if oxygen is present it can compete with –NCO by oxidizing primary amines and giving quinone type colored species seen at 380 nm in UV. This is especially true when secondary amines are formed, which react slower with -NCO and faster with oxygen in comparison to primary amines. In the model experiments with aniline, we found that Fyrol FR-2 and Fyrol PNX tend to N-alkylate primary amines producing secondary amines and thus, probably, accelerating scorch. Phosphoric acid species and HCl (only in the case of Fyrol FR-2) formed in the course of alkylation can further catalyze scorch. On the other hand, Fyrol PBR and AC003 do not react with aniline (primary amines) at the same conditions and therefore do not accelerate scorching. Because the oxidation of amines is the main cause of scorch in both flame retardant and non flame retardant foam, discoloration can be kept under control by addition of antioxidants. If foam is manufactured in very humid and hot environment non scorching flame retardants, e.g. AC003, is the better choice.