Characterization and Flame-Retardant Properties of Cobalt-Coordinated Cyclic Phosphonitrile in Thermoplastic Polyurethane Composites

Abstract

Halogen-free organophosphorus flame retardants have promising application prospects due to their excellent safety and environmental protection properties. A cobalt-coordinated cyclic phosphonitrile flame retardant (Co@CPA) was synthesized via a hydrothermal method using hexachlorocyclotriphosphonitrile (HCCP), 5-amino-tetrazolium (5-AT), and cobalt nitrate hexahydrate (Co(NO₃)2∙6H₂O) as starting materials. The structure was characterized using Fourier transform infrared (FTIR), nuclear magnetic resonance spectroscopy (1H-NMR), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA). Thermoplastic polyurethane (TPU) composites were prepared by incorporating 10-(2,5-dihydroxyphenyl)-9,10-dihydro-9-oxa-10-phosphame-10-oxide (ODOPB), Co@CPA, and silicon dioxide (SiO₂) via melt blending. The flame-retardant performance and thermal stability of the TPU composites were evaluated through limiting oxygen index (LOI), vertical combustion (UL-94), TG, and cone calorimetric (CCT) tests. SEM and Raman spectroscopy were used to analyze the surface morphology and structure of the residual carbon. A synergistic flame-retardant effect of ODOPB and Co@CPA was observed, with the most effective flame retardancy achieved at a TPU:ODOPB:Co@CPA:SiO₂ ratio of 75:16:8:1. This composition exhibited an LOI value of 26.5% and achieved a V-0 rating in the UL-94 test. Furthermore, compared to pure TPU, the composite showed reductions in total heat release, CO production, and CO₂ production by 6.6%, 39.4%, and 48.9%, respectively. Our research findings suggest that Co@CPA demonstrates outstanding performance, with potential for further expansion in application areas. Different metal-based cyclic phosphonitrile compounds are significant in enriching phosphorus-based fine chemicals.

Keywords: cobalt coordinated; hydrothermal; limiting oxygen index; metal-based cyclic phosphonitrile; organophosphorus.

Figure 1

Synthetic route of CPA.

Figure 2

Synthetic route of Co@CPA.

Figure 3

(a) FTIR spectra of CPA; (b) 1H NMR spectra of CPA.

Figure 4

(a) The SEM overall view of Co@CPA (b) and EDS element layer image, (c) Co kα, (d) C kα, (e) N kα, (f) P kα.

Figure 5

TGA and DTG curves of Co@CPA.

Figure 6

TGA (a) and DTG (b) curves of TPU and TPU composites.

Figure 7

(a) HRR, (b) THR and (c) weight loss ratio curves of TPU and TPU composites.

Figure 8

Photo of carbon residue after CCT: (a1,a2) TPU0, (b1,b2) TPU3, and (c1,c2) TPU4.

Figure 9

SEM images of the char residues after CCT for TPU3 (a) and TPU4 (b).

Figure 10

Raman spectra of the char residues from TPU3 (a) and TPU4 (b).

Figure 11

Mechanical properties of TPU and TPU composites (a) Tenslle strength, (b) Tenslle strain rate at break.