The preparation and properties of flame-retardant polyisocyanurate–polyurethane foams based on two DOPO derivatives

Two types of DOPO (9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide)-based halogen-free flame retardants, ODOPB (2-(6-oxid-6H-dibenzo[c,e][1,2]oxaphosphorin-6-yl)-1,4-benzenediol) and DOPO-PhOH (1,10-bis(4-hydroxyphenyl)-methylene-bispheny-1-oxophosphine oxide), were synthesized and incorporated into polyisocyanurate–polyurethane foams to investigate their effects on the thermal stability and flammability of polyisocyanurate–polyurethane foams. The thermal properties of the foams were determined using thermogravimetric analysis. The results showed that when compared to the polyisocyanurate–polyurethane control sample, the onset degradation temperatures of the foams were the same and the residues improved upon the addition of ODOPB, while for the polyisocyanurate–polyurethane/DOPO-PhOH foams, the onset degradation temperature and the residues were both improved. Furthermore, the physical–mechanical properties, flame retardant behavior, and morphological properties of foams were studied using thermal conductivity tests, compressive strength tests, the limiting oxygen index, cone calorimeter, and scanning electron microscopy. It was confirmed that the polyisocyanurate–polyurethane/DOPO derivative foams had the best physical–mechanical properties and flame retardancy compared to the polyisocyanurate–polyurethane/dimethyl methylphosphonate foam with the same phosphorus content added. The introduction of ODOPB caused a deterioration of the compression strength. However, the compression strength did not change upon the addition of DOPO-PhOH. The limiting oxygen index value showed a linear relationship with the phosphorous content in the foams upon the addition of ODOPB or DOPO-PhOH. Moreover, upon the addition of 20 phr ODOPB, the peak heat release rate of the foam was 25% lower than the polyisocyanurate–polyurethane control sample, while the reduced rate of the polyisocyanurate–polyurethane/20 phr DOPO-PhOH sample was 35%. In addition, for both the polyisocyanurate–polyurethane/ODOPB and polyisocyanurate–polyurethane/DOPO-PhOH foams, a continuous covering formed on the surface of residues was observed, which would obstruct heat and combustible gases from being transferred to the unburned foams.