Abstract
Three novel alkyl sulfone bridged phosphorus (P) compounds namely 6,6′-(sulfonylbis(ethane-2,1-diyl))bis(dibenzo[c,e][1,2]oxaphosphinine 6-oxide) (SEDOPO), sulfonylbis(ethane-2,1-diyl))bis(diphenylphosphine oxide) (SEDPPO), and tetraphenyl (sulfonyl bis(ethane-2,1-diyl)) bis(phosphonate) (SEDPP) (i.e. phosphine oxide, phosphinate and phosphonate, respectively) were synthesized via a Michael addition reaction with good yields (≥ 85%) at a 200-g scale. They exhibited thermal stability above 250 °C, which allowed them to be melt-processed with polypropylene (PP) and formed into thin films (~ 0.6 mm). Rheological measurements of the PP blends exhibited a typical shear thinning behavior and provided evidence for the synthesized compound's thermo-oxidative stabilizing effect. This was also confirmed by thermal analysis showing that the thermo-oxidative stability of PP-SEDOPO and PP-SEDPPO blends was higher (~25 °C) than the blank PP; however, PP-SEDPP had a smaller impact. Small scale fire tests of the PP-FR blends confirmed the flame retardant efficacy of the new P-compounds. Cone calorimetry on PP-SEDOPO blends showed a reduction in the heat release rate (HRR) (~48%) compared to blank PP. Further thermal and evolved gas analysis of the PP blends confirmed that the new P-compounds are primarily active in the gas-phase.
Highlights
Combustible due to its aliphatic chemical composition [4]
SEDPPO has been synthesized in solvent-free conditions in an alternative synthesis strategy by reacting one equivalent of divinyl sulfone and two equivalents of diphenylphosphine oxide using a microwaveassisted double-hydrophosphorylation of alkenes on a 50 mg scale [45]
The rheological measurements of the flame retardant (FR)/PP blends show that a higher viscosity was achieved for the blends; the degradation mechanism during processing becomes less severe with the bridged sulfone compounds
Summary
Combustible due to its aliphatic chemical composition [4]. Combustion of PP yields high amounts of toxic gases that threaten human safety and restrict its usage in fire-safe applications [5,6]. Blends of halogenated compounds with antimony trioxide (Sb2O3) can be used as FRs for PP; the blend can either produce toxic gases in combustion [10] or are themselves toxic [11] Inorganic flame retardants such as aluminum hydroxide Al(OH) and magnesium hydroxide (Mg(OH) are used as FR for PP, but their high loading requirements (~40 wt%) can result in materials with inferior rheological and mechanical properties [12,13,14,15] and are not suitable for film and fiber fabrication. N-alkoxy hindered amines (NORs) are commonly used in fiber and thin-film applications [16,17,18,19,20] They possess excellent flame retardancy due to the dissociation of NORs forming free radical scavengers that interrupt and suppress the ignition process of polyolefins [21,22]. Based on the various thermal and evolved gas analysis, a tentative mode of action of the bridged sulfone compounds was proposed
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