Posted on 15/10/2019 in 2019
Phosphorus FRs

Sabyasachi Gaan, EMPA Switzerland, presented development of DOPO derivative phosphorus PIN FRs for use in synthetic and bio-based polymers, from research through to industrial production: DOPO phosphphonamides (EDA-DOPO = 6,6′-(Ethane-1,2-diylbis(azanediyl))bis(dibenzo[c,e][1,2]oxaphosphinine-6-oxide)), today REACH registered and entering commercialisation by Metadynea; DOPO-PEPA (PEPA = 1-oxo-4-hydroxymethyl-2,6,7-trioxa-l-phosphabicyclo[2.2.2] octane), REACH registration underway, for application in polyester and polyolefins, under development with JET-Aviation for non-halogenated fire safety of wood products for aviation applications; and triazine DOPO, under development with BRUAG. He emphasised the importance of toxicity and ecotoxicity testing at the start of development. Some DOPO derivatives show significant aquatic toxicity or neurotoxicity, and these were discarded from development. Effective dispersion in polyols is also essential to enable processing.

Manfred Döring, Fraunhofer LBF, summarised developments in chemistry and applications for DOPO derivatives as phosphorus based PIN flame retardants. DOPO ( DOPO = 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide ) mainly acts in the gas phase, by releasing phosphorus radicals which inhibit the flame, but chemical modifications of DOPO can increase the solid phase (char formation) fire protection effects. The reactive P-H bond in DOPO is reactive, enabling versatile chemistries and the development of molecules with specific properties. For example, DOPO containing hardeners have been developed for epoxies, thus reacting the phosphorus PIN FR into the epoxy polymer, or modifications of DOPO and incorporation or other molecules or atoms can improve impact of DOPO addition on polymer performance, improve char generation or improve thermal stability and so processing compatibility.

Roland Krämer, BASF, presented results from micro-scale flame calorimeter testing of nearly 200 samples of polyamide and PBT with phosphorus-based PIN FRs, with the objective of understanding which loadings of these flame retardants are necessary to achieve required levels of fire performance (UL94-V0, 0.8 mm and 1.6 mm). Although single fire properties (e.g. heat release) were statistically correlated to UL94-V0 pass, single parameters show a too high scattering for practical predictions. However, by using several fire properties determined by calorimetric techniques, robust limits for pass/fail can be determined.

Yuan Hu, China State Key Laboratory of Fire Science, Anhui, presented synthesis of polyphosphazenes (indifferent nano-forms), with integration of silicon, cobalt and copper, and used for micro-encapsulation of ammonium polyphosphate. These nano-form PINs were tested as additive FRs in epoxies at up to 10% loading. Results showed significantly improved fire performance (e.g. heat release rates) and reduced emissions of both smoke and toxic gases (carbon monoxide, hydrocarbons, aromatics, carbonyls), probably because the PIN nano-composites were acting to catalyse redox reactions to complete oxidation of partial combustion gases.

Alexander Morgan, University of Dayton, USA, presented tests of phosphorus hydrazides as reactive PIN FRs in epoxies. These react into the epoxy during epoxy polymerisation, so providing non migrating, in depth fire protection, by both charring and nitrogen release. Tests showed that this reactive PIN FR solution significantly reduced smoke emissions. Objectives are also to enable compatibility with epoxy electrical performance (printed circuit boards) and with corrosion resistance, and prevent re-ignition after fires (epoxies retain heat). A challenge may be scale-up of production, because the hydrazines can interfere with epoxy polymerisation and both chemistry and processing may need some adaptation.

Johannes Lenz, Leibnitz Institute for Polymer Research, Dresden, presented a new proposed phosphorus-based PIN FR, the cyclic phosphonate BPPO and derivatives (dibenzo[d,f][1,3,2]dioxa phosphapepine 6-oxide). BPPO can be simply synthesised in one step from three-component condensation. The substance’s aromatic ring is highly reactive, enabling a gas phase FR action. Phosphorus content can be increased in BPPO derivatives, by reactions with acrylates and diesters. These were tested as a PIN FR in rigid PUR/PIR foams, showing good dispersion, no significant impact on foam characteristics and fire performance comparable or better than conventional FR foams (TPP triphenyl phosphate) at comparable phosphorus contents.

Andrea Toldy, Budapest University of Technology and Economy, presented testing of two phosphorus-based PIN FRs (APP and RDP) in carbon-fibre reinforced epoxy resin, at phosphorus loadings of 0 – 5%, showing synergistic flame retardancy effects. A challenge was the dispersion of solid APP, with tendency to accumulate in upper layers of the composite produced by RTM (Resin Transfer Molding), which led to lower pHRR and higher residue during Mass Loss Calorimeter tests.

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