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Both alkylphosphinates and inorganic phosphinates (based on sodium, calcium, magnesium or zinc) have been recently proposed as flame retardants for polyesters, polyamides and polyurethane foams as well. The main aim of this work was to compare the flame retardant effectiveness of inorganic (already proofed in PU foams) and organic phosphinates in PU foams which have never been used in polyurethane (PU) foams. The thermal stability in nitrogen and air as well as limiting oxygen index and cone calorimeter behaviour have been studied to assess the effectiveness of such flame retardants in PU foams.
The results obtained showed that both inorganic and organic phosphinates are effective in enhancing fire behaviour of PU foams since they improve thermal stability, LOI and fire performance. Cone calorimetry highlighted the flame inhibition action in the gas phase due to the release of phosphorus-containing molecules. The better results obtained for inorganic phosphinate are probably related to the better quality of the char layer developed during burning, but may also be related to the higher phosphorus content of such flame retardant with respect the other ones. It was also verified that both inorganic and organic phosphinate containing N-synergic compound showed a fuel dilution effect, deriving from water and/or ammonia release in the gas phase.
Nanocomposites of a charring polymer (like polyurethane foam) filled with aluminum phosphinate (AlPi) with or without melamine cyanurate (MelCy) have been prepared by microwave processing and their thermal stability and fire behavior have been studied. Results on the interaction between flame retardants and layered silicates were provided as well as detailed investigation of the char strength, which has been carried out using a suitably developed method based on dynamic-mechanic analysis.
Generally, the thermo-oxidative stability in presence of layered silicates was higher than the counterparts even if an additive rather than synergic effect took place; however, in some cases the interaction between clays and phosphinate led to a significant decrease of weight residue. In nitrogen the residue amounts were about the same but a higher amount of phosphorus was retained in the solid phase in presence of clays. Cone calorimeter results showed that the use of phosphinates led to a decrease of the PHRR; further addition of clays did not reduce the PHRR owing to the worse quality of char layer as demonstrated by the char strength test. However, it has been shown that the partial substitution of aluminum phosphinate with melamine cyanurate gave improved results: the AlPi–MelCy filled foams showed similar pHRR and THE but lower TSR and higher char strength than AlPi filled foams. It was also confirmed that phosphinate acted by flame inhibition but its action was depressed by the use of nanoclays owing to their interaction.
Several expandable graphites (EGs), differing in Expansion volume but with the same mean size, are compared as flame retardants in polyurethane (PUR) foams. Not only common sulfur-intercalated graphites are investigated but also a new one intercalated with phosphorus. The main aim of this article is to understand which properties of EG are important for its flame retardancy effectiveness in PUR foams. Thermal stability, flammability, and fire behavior are analyzed through limiting oxygen index and cone calorimeter tests. Detailed characterization of the phosphorus-intercalated graphite is also provided as well as physical–mechanical characterization. The results show that the well-known sulfur-intercalated graphites and the one with phosphorus both enhance the residue yield, induce a protective layer, and thus efficiently flame-retard PUR foams. While the expansion volume of the EGs had a surprisingly limited influence on the performance of the foams, at least in the range tested, the most important feature Controlling the effectiveness of EG in terms of flame retardant PUR foams was the type of intercalant. The presence of EG affected the physical–mechanical properties of the foams; however, no significant effect of the expansion volume or intercalant type has been revealed on the physical–mechanical properties of the foams.