Skip to main content

A core–shell-structured APP@COFs hybrid for enhanced flame retardancy and mechanical property of epoxy resin (EP)


Epoxy resin (EP) is a commercially important resin with many important industrial applications but is impeded by its inherent flammability. Ammonium polyphosphate (APP) represents an eco-friendly and effective fire retardant for EP, but its moisture sensitivity and poor interfacial compatibility with EP often give rise to unsatisfactory fire retardance and adverse impacts on mechanical properties of EP. To address these issues, we herein report a core–shell-structured modified APP, APP@COFs, using Schiff base covalent organic frameworks (COFs) as a surface modifier. The results show that the addition of 2 parts per hundreds of resins (phr) APP@COFs effectively enhances the flame retardancy of EP, leading to a self-extinguishing capability and a limiting oxygen index of 27.1%. Compared with virgin EP, the peak heat release rate is decreased by 54.7% due to the modes of action of APP@COFs in both gas and condensed phases. Additionally, because of improved interfacial compatibility, the resulting EP/APP@COFs composites show improved mechanical properties, e.g., a 37% increase in the impact toughness of EP/2 phr APP@COFs relative to that of EP. This work provides an effective method for modifying APP and the development of advanced fire-retardant EP materials for practical applications in industries.

Graphical abstract

A core–shell-structured fire retardant has been developed for creating fire-retardant epoxy resin composites with improved mechanical properties.

This is a preview of subscription content, access via your institution.

Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
Fig. 7
Fig. 8


  1. Lou G, Ma Z, Dai J, Bai Z, Fu S, Huo S, Qian L, Song P (2021) Fully biobased surface-functionalized microcrystalline cellulose via green self-assembly toward fire-retardant, strong, and tough epoxy biocomposites. ACS Sustain Chem Eng 9:13595–13605

    CAS  Article  Google Scholar 

  2. Wang G, Nie Z (2016) Synthesis of a novel phosphorus-containing epoxy curing agent and the thermal, mechanical and flame-retardant properties of the cured products. Polym Degrad Stab 130:143–154

    CAS  Article  Google Scholar 

  3. Yang S, Huo S, Wang J, Zhang B, Wang J, Ran S, Fang Z, Song P, Wang H (2021) A highly fire-safe and smoke-suppressive single-component epoxy resin with switchable curing temperature and rapid curing rate. Compos Part B-Eng 207:108601

  4. Zhang Y, Jing J, Liu T, Xi L, Sai T, Ran S, Fang Z, Huo S, Song P (2021) A molecularly engineered bioderived polyphosphate for enhanced flame retardant, UV-blocking and mechanical properties of poly(lactic acid). Chem Eng J 411:128493

  5. Sai T, Ran S, Guo Z, Yan H, Zhang Y, Wang H, Song P, Fang Z (2021) Transparent, highly thermostable and flame retardant polycarbonate enabled by rod-like phosphorous-containing metal complex aggregates. Chem Eng J 409:128223

  6. Qiu S, Ma C, Wang X, Zhou X, Feng X, Yuen RKK, Hu Y (2018) Melamine-containing polyphosphazene wrapped ammonium polyphosphate: a novel multifunctional organic-inorganic hybrid flame retardant. J Hazard Mater 344:839–848

    CAS  Article  Google Scholar 

  7. Liang C, Du Y, Wang Y, Ma A, Huang S, Ma Z (2021) Intumescent fire-retardant coatings for ancient wooden architectures with ideal electromagnetic interference shielding. Adv Compos Hybrid Mater 4:979–988

    CAS  Article  Google Scholar 

  8. Liu L, Xu Y, Li S, Xu M, He Y, Shi Z, Li B (2019) A novel strategy for simultaneously improving the fire safety, water resistance and compatibility of thermoplastic polyurethane composites through the construction of biomimetic hydrophobic structure of intumescent flame retardant synergistic system. Compos Part B-Eng 176:107218

  9. Fang F, Song P, Ran S, Guo Z, Wang H, Fang Z (2018) A facile way to prepare phosphorus-nitrogen-functionalized graphene oxide for enhancing the flame retardancy of epoxy resin. Compos Commun 10:97–102

    Article  Google Scholar 

  10. Xue Y, Shen M, Zeng S, Zhang W, Hao L, Yang L, Song P (2019) A novel strategy for enhancing the flame resistance, dynamic mechanical and the thermal degradation properties of epoxy nanocomposites. Mater Res Express 6:125003

  11. Guo J, Chen Z, Abdul W, Kong J, Khan MA, Young DP, Zhu J, Guo Z (2021) Tunable positive magnetoresistance of magnetic polyaniline nanocomposites. Adv Compos Hybrid Mater 4:534–542

    CAS  Article  Google Scholar 

  12. Guo J, Li X, Chen Z, Zhu J, Mai X, Wei R, Sun K, Liu H, Chen Y, Naik N, Guo Z (2022) Magnetic NiFe2O4/polypyrrole nanocomposites with enhanced electromagnetic wave absorption. J Mater Sci Technol 108:64–72

    Article  Google Scholar 

  13. Guo J, Li X, Liu H, Young DP, Song G, Song K, Zhu J, Kong J, Guo Z (2021) Tunable magnetoresistance of core-shell structured polyaniline nanocomposites with 0-, 1-, and 2-dimensional nanocarbons. Adv Compos Hybrid Mater 4:51–64

    CAS  Article  Google Scholar 

  14. Zhang D, Williams BL, Santos VH, Lofink BJ, Becher EM, Partyka A, Peng X, Sun L (2020) Self-assembled intumescent flame retardant coatings: influence of pH on the flammability of cotton fabrics. Eng Sci 12:106–112

    CAS  Google Scholar 

  15. Ding SY, Gao J, Wang Q, Zhang Y, Song WG, Su CY, Wang W (2011) Construction of covalent organic framework for catalysis: Pd/COF-LZU1 in Suzuki-Miyaura coupling reaction. J Am Chem Soc 133:19816–19822

    CAS  Article  Google Scholar 

  16. Chen L, Furukawa K, Gao J, Nagai A, Nakamura T, Dong Y, Jiang D (2014) Photoelectric covalent organic frameworks: converting open lattices into ordered donor-acceptor heterojunctions. J Am Chem Soc 136:9806–9809

    CAS  Article  Google Scholar 

  17. Mulzer CR, Shen L, Bisbey RP, McKone JR, Zhang N, Abruna HD, Dichtel WR (2016) Superior charge storage and power density of a conducting polymer-modified covalent organic framework. ACS Cent Sci 2:667–673

    CAS  Article  Google Scholar 

  18. Shinde DB, Kandambeth S, Pachfule P, Kumar RR, Banerjee R (2015) Bifunctional covalent organic frameworks with two dimensional organocatalytic micropores. Chem Comm 51:310–313

    CAS  Article  Google Scholar 

  19. Zeng Y, Zou R, Zhao Y (2016) Covalent organic frameworks for CO2 capture. Adv Mater 28:2855–2873

    CAS  Article  Google Scholar 

  20. Chen X, Addicoat M, Jin E, Xu H, Hayashi T, Xu F, Huang N, Irle S, Jiang D (2015) Designed synthesis of double-stage two-dimensional covalent organic frameworks. Sci Rep 5:14650

    CAS  Article  Google Scholar 

  21. Feng X, Chen L, Dong Y, Jiang D (2011) Porphyrin-based two-dimensional covalent organic frameworks: synchronized synthetic control of macroscopic structures and pore parameters. Chem Comm 47:1979–1981

    CAS  Article  Google Scholar 

  22. Xiao Y, Jin Z, He L, Ma S, Wang C, Mu X, Song L (2020) Synthesis of a novel graphene conjugated covalent organic framework nanohybrid for enhancing the flame retardancy and mechanical properties of epoxy resins through synergistic effect. Compos Part B-Eng 182:107616

  23. Zhu SR, Qi Q, Fang Y, Zhao WN, Wu MK, Han L (2018) Covalent triazine framework modified BiOBr nanoflake with enhanced photocatalytic activity for antibiotic removal. Cryst Growth Des 18:883–891

    CAS  Article  Google Scholar 

  24. Ma W, Zheng Q, He Y, Li G, Guo W, Lin Z, Zhang L (2019) Size-controllable synthesis of uniform spherical covalent organic frameworks at room temperature for highly efficient and selective enrichment of hydrophobic peptides. J Am Chem Soc 141:18271–18277

    CAS  Article  Google Scholar 

  25. Sun Q, Aguila B, Perman J, Earl LD, Abney CW, Cheng Y, Wei H, Nguyen N, Wojtas L, Ma S (2017) Postsynthetically modified covalent organic frameworks for efficient and effective mercury removal. J Am Chem Soc 139:2786–2793

    CAS  Article  Google Scholar 

  26. Jiang M, Zhang Y, Yu Y, Zhang Q, Huang B, Chen Z, Chen T, Jiang J (2019) Flame retardancy of unsaturated polyester composites with modified ammonium polyphosphate, montmorillonite, and zinc borate. J Appl Polym Sci 136:47180

    Article  Google Scholar 

  27. Feng X, Fan J, Li A, Li G (2019) Multireusable thermoset with anomalous flame-triggered shape memory effect. ACS Appl Mater Inter 11:16075–16086

    CAS  Article  Google Scholar 

  28. Nie S, Hu Y, Song L, He Q, Yang D, Chen H (2008) Synergistic effect between a char forming agent (CFA) and microencapsulated ammonium polyphosphate on the thermal and flame retardant properties of polypropylene. Polym Adv Technol 19:1077–1083

    CAS  Article  Google Scholar 

  29. Jin X, Sun J, Zhang JS, Gu X, Bourbigot S, Li H, Tang W, Zhang S (2017) The preparation of a novel intumescent flame retardant based on supramolecular interactions and its application in polyamide 11. ACS Appl Mater Inter 9:24964–24975

    CAS  Article  Google Scholar 

  30. Shao ZB, Deng C, Tan Y, Chen MJ, Chen L, Wang YZ (2014) An efficient mono-component polymeric intumescent flame retardant for polypropylene: preparation and application. ACS Appl Mater Inter 6:7363–7370

    CAS  Article  Google Scholar 

  31. Sun Y, Yuan B, Chen X, Li K, Wang L, Yun Y, Fan A (2019) Suppression of methane/air explosion by kaolinite-based multi-component inhibitor. Powder Technol 343:279–286

    CAS  Article  Google Scholar 

  32. Shao ZB, Deng C, Tan Y, Yu L, Chen MJ, Chen L, Wang YZ (2014) Ammonium polyphosphate chemically-modified with ethanolamine as an efficient intumescent flame retardant for polypropylene. J Mater Chem 2:13955

    CAS  Article  Google Scholar 

  33. Chen Y, Li L, Xu L, Qian L (2018) Phosphorus-containing silica gel-coated ammonium polyphosphate: preparation, characterization, and its effect on the flame retardancy of rigid polyurethane foam. J Appl Polym Sci 135:46334

    Article  Google Scholar 

  34. Wang W, Deng S, Ren L, Li D, Wang W, Vakili M, Wang B, Huang J, Wang Y, Yu G (2018) Stable covalent organic frameworks as efficient adsorbents for high and selective removal of an aryl-organophosphorus flame retardant from water. ACS Appl Mater Inter 10:30265–30272

    CAS  Article  Google Scholar 

  35. Liu Y, Xu W, Chen R, Cheng C, Hu Y (2020) Effect of different zeolitic imidazolate frameworks nanoparticle-modified β-FeOOH rods on flame retardancy and smoke suppression of epoxy resin. J Appl Polym Sci 138:49637

    Article  Google Scholar 

  36. Guo W, Yu B, Yuan Y, Song L, Hu Y (2017) In situ preparation of reduced graphene oxide/DOPO-based phosphonamidate hybrids towards high-performance epoxy nanocomposites. Compos Part B-Eng 123:154–164

    CAS  Article  Google Scholar 

  37. Qiu S, Zou B, Zhang T, Ren X, Yu B, Zhou Y, Kan Y, Hu Y (2020) Integrated effect of NH2-functionalized/triazine based covalent organic framework black phosphorus on reducing fire hazards of epoxy nanocomposites. Chem Eng J 401:126058

  38. Bi X, Meng W, Meng Y, Di H, Li J, Xie J, Xu J, Fang L (2021) Novel [BMIM]PF6 modified flake-ANP flame retardant: synthesis and application in epoxy resin. Polym Test 101:107284

  39. Meng W, Dong Y, Li J, Cheng L, Zhang H, Wang C, Jiao Y, Xu J, Hao J, Qu H (2020) Bio-based phytic acid and tannic acid chelate-mediated interfacial assembly of Mg(OH)2 for simultaneously improved flame retardancy, smoke suppression and mechanical properties of PVC. Compos Part B-Eng 188:107854

  40. Decsov K, Bocz K, Szolnoki B, Bourbigot S, Fontaine G, Vadas D, Marosi G (2019) Development of bioepoxy resin microencapsulated ammonium-polyphosphate for flame retardancy of polylactic acid. Molecules 24:4123

    CAS  Article  Google Scholar 

  41. Deng CL, Deng C, Zhao J, Fang WH, Lin L, Wang YZ (2014) Water resistance, thermal stability, and flame retardation of polypropylene composites containing a novel ammonium polyphosphate microencapsulated by UV-curable epoxy acrylate resin. Polym Adv Technol 25:861–871

    CAS  Article  Google Scholar 

  42. Xue Y, Feng J, Huo S, Song P, Yu B, Liu L, Wang H (2020) Polyphosphoramide-intercalated MXene for simultaneously enhancing thermal stability, flame retardancy and mechanical properties of polylactide. Chem Eng J 397:125336

  43. Liu L, Zhu M, Ma Z, Xu X, Mohesen Seraji S, Yu B, Sun Z, Wang H, Song P (2022) A reactive copper-organophosphate-MXene heterostructure enabled antibacterial, self-extinguishing and mechanically robust polymer nanocomposites. Chem Eng J 430:132712

  44. Kang X, Lu Z, Feng W, Wang J, Fang X, Xu Y, Wang Y, Liu B, Ding T, Ma Y, Pan D, Patil RR, Murugadoss V (2021) A novel phosphorous and silicon-containing benzoxazine: highly efficient multifunctional flame-retardant synergist for polyoxymethylene. Adv Compos Hybrid Mater 4:127–137

    CAS  Article  Google Scholar 

  45. Meng WH, Wu HJ, Bi X, Huo ZY, Wu JN, Jiao YH, Xu JZ, Wang M, Qu HQ (2021) Synthesis of ZIF-8 with encapsulated hexachlorocyclotriphosphazene and its quenching mechanism for flame-retardant epoxy resin. Micropor Mesopor Mat 314:110885

  46. Zhu M, Liu L, Wang Z (2020) Iron-phosphorus-nitrogen functionalized reduced graphene oxide for epoxy resin with reduced fire hazards and improved impact toughness. Compos Part B-Eng 199:108283

  47. Fan S, Sun Y, Wang X, Yu J, Wu D, Li F (2020) A novel organic-inorganic flame retardant of ammonium polyphosphate chemically coated by Schiff base-containing branched polysiloxane for polyamide 6. Polym Adv Technol 31:2763–2774

    CAS  Article  Google Scholar 

  48. Ding S, Liu P, Zhang S, Ding Y, Wang F, Gao C, Yang M (2020) Preparation and characterization of cyclodextrin microencapsulated ammonium polyphosphate and its application in flame retardant polypropylene. J Appl Polym Sci 137:49001

    CAS  Article  Google Scholar 

  49. Ma Z, Zhang J, Maluk C, Yu Y, Seraji S, Yu B, Wang H, Song P (2022) A lava-inspired micro/nano-structured ceramifiable organic-inorganic hybrid fire-extinguishing coating. Matter 5

  50. Zhang Y, Xiong Z, Ge H, Ni L, Zhang T, Huo S, Song P, Fang Z (2020) Core-shell bioderived flame retardants based on chitosan/alginate coated ammonia polyphosphate for enhancing flame retardancy of polylactic acid. ACS Sustain Chem Eng 8:6402–6412

    CAS  Article  Google Scholar 

  51. Luo F, Wu K, Li Y, Zheng J, Guo H, Lu M (2015) Reactive flame retardant with core-shell structure and its flame retardancy in rigid polyurethane foam. J Appl Polym Sci 132:42800

    Article  Google Scholar 

  52. Qi C, Yuan B, Dong H, Li K, Shang S, Sun Y, Chen G, Zhan Y (2020) Supramolecular self-assembly modification of ammonium polyphosphate and its flame retardant application in polypropylene. Polym Adv Technol 31:1099–1109

    CAS  Article  Google Scholar 

  53. Liu L, Xu Y, Pan Y, Xu M, Di Y, Li B (2021) Facile synthesis of an efficient phosphonamide flame retardant for simultaneous enhancement of fire safety and crystallization rate of poly (lactic acid). Chem Eng J 421:127761

  54. Kim M, Ko H, Park SM (2019) Synergistic effects of amine-modified ammonium polyphosphate on curing behaviors and flame retardation properties of epoxy composites. Compos B Eng 170:19–30

    CAS  Article  Google Scholar 

  55. Yang K, Xu M-J, Li B (2013) Synthesis of N-ethyl triazine-piperazine copolymer and flame retardancy and water resistance of intumescent flame retardant polypropylene. Polym Degrad Stab 98:1397–1406

    CAS  Article  Google Scholar 

  56. Xu X, Dai J, Ma Z., Liu L, Zhang X, Liu H. Tang L, Huang G, Wang H, Song P (2020) Manipulating interphase reactions for mechanically robust, flame-retardant and sustainable polylactide biocomposites. Compos Part B-Eng 190:107930

Download references


This work was financially supported by the Key Research and Development Projects of Hebei Province (No. 19211205D), the Postdoctoral Fund of Hebei University (No. 703202105), the Multidisciplinary Research Project of Hebei University (No. DXK202003), and the Post-graduate’s Innovation Fund Project of Hebei University (No. HBU2021ss019).

Author information

Authors and Affiliations


Corresponding authors

Correspondence to Weihua Meng, Pingan Song or Jianzhong Xu.

Ethics declarations

Conflict of interest

The authors declare no competing interests.

Additional information

Publisher's Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary Information

Below is the link to the electronic supplementary material.

Supplementary file1 (DOCX 454 kb)

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Bi, X., Di, H., Liu, J. et al. A core–shell-structured APP@COFs hybrid for enhanced flame retardancy and mechanical property of epoxy resin (EP). Adv Compos Hybrid Mater 5, 1743–1755 (2022).

Download citation

  • Received:

  • Revised:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI:


  • Epoxy resin
  • Covalent organic frameworks
  • Ammonium polyphosphate
  • Flame retardancy
  • Mechanical property