Influence of Phosphorus Structures and Their Oxidation States on Flame-Retardant Properties of Polyhydroxyurethanes

Abstract

This article focuses on the synthesis of polyhydroxyurethane (PHU) materials containing novel phosphorus flame retardants (FR). Four different phosphorus compounds were grafted onto cyclic carbonate: 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), diethyl phosphite (DEP), diphenyl phosphite (DPP) and dibenzo[d,f][1,3,2]dioxaphosphepine 6-oxide (BPPO). Thus, three novel phosphorus reactive cyclic carbonates which have never been reported so far were synthetized. Phosphorus FR containing PHU materials were characterized by FTIR to evidence the total conversion of the cyclic carbonate. Moreover, the gel contents up to 80% confirmed the formation of the polymer network. Then, the thermal stability and the flame-retardant properties were investigated by thermogravimetric analyses, cone calorimeter and pyrolysis combustion flow calorimeter. The mode of action of phosphorus compounds, depending on the oxidation state, was especially highlighted. Phosphonate (+III) provided better action in a condensed phase than phosphinate thanks to a more efficient char formation. Among phosphonates, differences were observed in terms of char-formation rate and expansion. DEP provided the best flame-retardant properties, with a reduction of 76% of pHRR with 2 wt% of phosphorus in cone calorimeter analysis. Therefore, this article highlighted the different modes of action of phosphorus flame retardants, depending on the oxidation state of phosphorus, in PHU materials.

How Do Phosphorus Compounds with Different Valence States Affect the Flame Retardancy of PET?

“…This phenomenon proved that +3-valence phosphorus can release more PO and PO 2 free radicals in the gas phase, so as to play a stronger role in gas-phase action [24,28]. During the combustion, C=O (1762 cm −1 ) and hydrocarbon (1280-1470 cm −1 ) fragments were usually released into the gas phase as volatile combustible fuels to sustain the combustion process [29]. From Figure 7, we can find that the absorption peaks of hydrocarbon and C=O significantly decreased with the increase of +5-valence phosphorus content in polyphosphonates; Figure 7f more clearly shows this trend, which indicated that, due to the high charring effect of +5-valence phosphorus, more flammable hydrocarbons were retained in the condensed phase and reduced the release of fuels, thus suppressing the combustion of composites.…”

From phosphanylphosphaalkenes to coordination copper and silver polymers containing P–P bonds

“…[40][41][42][43][44] Enrichment of the polymer chain with phosphorus atoms significantly changes the physical properties, e.g., decreases flammability of or gives greater flexibility to the product. 45,46 Moreover, a well-accessible P-P bond in the polymers carries many possibilities for further application. It has been shown that this kind of bond can serve as an assimilation site for small molecules such as CO 2 .…”