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Phosphorylated Salicylic Acid as Flame Retardant in Epoxy Resins and Composites

Lara Greiner1,*, Philipp Kukla2, Sebastian Eibl1, Manfred Döring3

1 Bundeswehr Research Institute for Materials, Fuels and Lubricants, Erding, 85435, Germany
2 Fraunhofer Institute for Structural Durability and System Reliability LBF, Darmstadt, 64289, Germany
3 Schill+Seilacher GmbH, Böblingen, 71032, Germany

* Corresponding Author: Lara Greiner. Email: email

(This article belongs to this Special Issue: Bio-based Halogen-free Flame Retardant Polymeric Materials)

Journal of Renewable Materials 2022, 10(7), 1931-1950.


A novel, versatile flame retardant substructure based on phosphorylated salicylic acid (SCP) is described and used in the synthesis of new flame retardants for HexFlow® RTM6, a high-performance epoxy resin used in resin transfer molding processes as composite matrix. The starting material salicylic acid can be obtained from natural sources. SCP as reactive phosphorus chloride is converted with a novolak, a novolak containing 9, 10-dihydro-9- oxa-10-phospha-phenanthrene-10-oxide (DOPO) substituents or DOPO-hydroquinone to flame retardants with sufficient thermal stability and high char yield. Additionally, these flame retardants are soluble in the resin as well as react into the epoxy network. The determined thermal stability and glass transition temperatures of flame retarded neat resin samples as well as the interlaminar shear strength of corresponding carbon fiber reinforced composite materials showed the applicability of these flame retardants. Neat resin samples and composites were tested for their flammability by UL94 and/or flame-retardant performance by cone calorimetry. All tested flame retardants decrease the peak of heat release rate by up to 54% for neat resin samples. A combination of DOPO and SCP in one flame retardant shows synergistic effects in char formation and the mode of action adapts to neat resin or fiber-reinforced samples, so there is efficient flame retardancy in both cases. Therefore, a tailoring of SCP based flame retardants is possible. Additionally, these flame retardants efficiently reduce fiber degradation during combustion of carbon fiber-reinforced epoxy resins as observed by scanning electron microscopy and energy dispersive X-ray spectroscopy.


Cite This Article

Greiner, L., Kukla, P., Eibl, S., Döring, M. (2022). Phosphorylated Salicylic Acid as Flame Retardant in Epoxy Resins and Composites. Journal of Renewable Materials, 10(7), 1931–1950.

cc This work is licensed under a Creative Commons Attribution 4.0 International License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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