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Phosphorus-containing flame retardants synthesized from renewable resources have had a lot of impact in recent years. This article outlines the synthesis, characterization and evaluation of these compounds in polyesters and epoxy resins. The different approaches used in producing biobased flame retardant polyesters and epoxy resins are reported. While for the polyesters biomass derived compounds usually are phosphorylated and melt blended with the polymer, biobased flame retardants for epoxy resins are directly incorporated into the polymer structure by a using a phosphorylated biobased monomer or curing agent. Evaluating the efficiency of the flame retardant composites is done by discussing results obtained from UL94 vertical burning, limiting oxygen index (LOI) and cone calorimetry tests. The review ends with an outlook on future development trends of biobased flame retardant systems for polyesters and epoxy resins.

Novel polymeric flame retardants based on two acrylamides and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) or 5,5-dimethyl-[1,3,2]dioxaphosphinane-2-oxide (DDPO) are described for several applications in HexFlow® RTM6, a high-performance epoxy resin. Neat resin samples and carbon fiber-reinforced composites were tested for their glass transition temperatures (dynamic mechanical analysis), thermal stability (thermogravimetric analyses), flammability (UL94) and flame-retardant performance (Cone Calorimetry). Additionally, the fiber degradation occurring during combustion of carbon fiber-reinforced epoxy resins was observed by scanning electron microscopy to show the fiber protecting effect of these flame retardants. Whereas DOPO-containing polyacrylamides acting mainly in the gas phase showed the best flame retardant efficiency, DDPO-containing polyacrylamides acting mainly in the condensed phase showed the best fiber protection. A mixed polyacrylamide was synthesized to combine these effects. This thermoplastic is soluble in the resin and, therefore, suitable for injection molding processes. Interlaminar shear strength measurements showed no negative effect of the flame retardant. The versatility of these flame retardants is shown by investigations dealing with boehmite as synergist in neat resin samples.

The influence of the specific surface area of aluminum trihydrate (ATH) on the synergism to different phosphorus‐based flame retardants (FRs) in unsaturated polyester (UP) resin was investigated. Flammability and thermal stability of UP composites were evaluated with UL‐94 vertical burning, thermogravimetric analysis, and cone calorimetry. The morphology of char residues was examined via scanning electron microscopy (SEM). An interaction between crystalline ATH (4.5 m2 g−1) and aluminum hypophosphite (AHP) or zinc diethylphosphinate (DEPZn) was observed. When 37 wt % ATH and 8 wt % AHP or DEPZn were incorporated, samples passed V‐0 in UL‐94 tests. Furthermore, peak of heat release rate (PHRR) and total heat release (THR) were reduced by introduction of phosphorus containing FRs compared to a formulation containing solely crystalline ATH. The incorporation of amorphous ATH (300 m2 g−1) leaded to lower UL‐94 ratings, using the same amount of AHP or DEPZn. In comparison to crystalline ATH, an addition of phosphorus containing FRs did not lead to a decrease of PHRR and THR for amorphous ATH. SEM of the residues showed that the combination of crystalline ATH with phosphorus species having hydrogen/carbon rich environment formed a dense and tough layer. The combination of crystalline ATH with phosphorus species providing oxygen rich environment gives fragile residues. An incorporation of amorphous ATH leaded to a fragile residue as well. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47270.

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