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Nov 04, 2024

Preparation and characterization of novel flame-retardant paint of substituted cyclodiphosph(V)azane sulfonomide and their Cu(II), Cd(II) metal complexes as new additives for exterior wood coating protection | Scientific Reports

Scientific Reports volume 14, Article number: 14452 (2024) Cite this article 657 Accesses Metrics details The development of flame-retardant materials has become an important research direction. For

Scientific Reports volume 14, Article number: 14452 (2024) Cite this article

657 Accesses

Metrics details

The development of flame-retardant materials has become an important research direction. For the past dozen years, researchers have been exploring flame retardants with high flame-retardant efficiency, low toxicity, less smoke, or other excellent performance flame retardants. Therefore, this work aimed to synthesize new cyclodiphosph(V)azane derivatives and their Cu(II) and Cd(II) metal complexes and investigated their potential applications as high flame-retardant efficiency. Various techniques were used to characterize the prepared ligand H2L and its metal complexes, including elemental analyses, mass spectra, conductivity measurements, electronic spectral data UV–vis, FT-IR, 1H,13C-NMR, TGA, XRD, and molecular docking experiments studies were M. tuberculosis receptors (PDB ID: 5UHF) and the crystal structure of human topoisomerase II alpha (PDB ID: 4FM9). Wood-based paint was physically mixed with the ligand H2L and its metal complexes. The obtained results of mechanical characteristics of the dried paint layers were noticed to improve, such as gloss value, which ranged from 85 to 95, hardness 1.5–2.5 kg, adhesion 4B to 5B, and impact resistance, which improved from 1.3 to 2.5 J. Moreover, the obtained results of flame-retardant properties showed a significant retardant impact compared to the blank sample, such as ignitability, which includes the heat flux which increased from 10 to 25 kW/m2, and ignition time, ranging from 550 to 1200 s, respectively, and limiting oxygen index (LOI) (%) which has been increased from 21 to 130 compared with the plywood sample and sample blank. The ordering activity of the observed results was noticed that coated sample based on Cd(II) metal complexes > coated sample based on Cu(II) metal complexes of Cyclophosphazene ligand > coated sample based on phosphazene ligand H2L > coated sample without additives > uncoated sample. This efficiency may be attributed to (1) the H2L is an organophosphorus compound, which contains P, N, Cl, and aromatic six- and five-member ring, (2) Cu(II) and Cd(II) metal complexes characterized by high thermal stability, good stability, excellent performance flame retardants, and wide application.

The structural features of four-membered N2P2 ring compounds in which the coordination number of P varies from three to five have attracted considerable attention1,2. Enhancing the thermal stability and flame retardancy of materials has been made possible by synthesizing several reactive or additive flame retardants by side-group substitution3,4,5. Moreover, flame retardants containing phosphorus and nitrogen have attracted much attention due to their high effectiveness and lack of toxicity. Furthermore, nitrogen and phosphorus can be combined to form more effective flame-retardant polymers to increase flame retardancy6. A few flame retardants based on nitrogen and phosphorus have been added to epoxy resin, and the compounds released after combustion are less hazardous7. Research on the potential applications of a recently discovered cross-linking polyphosphazene with active amine groups to reduce the fire hazard of epoxy resin was prompted by its successful synthesis8. Successful synthesis and characterization of hexasubstituted cyclotriphosphazene compounds with novel flame-retardant characteristics was achieved. Thermogravimetric analysis (TGA) and the limiting oxygen index were employed to investigate the flame-retardant property. Epoxy resin was used to create the molding matrix for this experiment9.

Phosphozenes and their derivatives have significant nitrogen and phosphorus contents, possibly contributing to their potential as flame retardants. Phosphazenes are superior building blocks for flame-retardant polymers10,11,12,13,14. They have been successfully used in epoxy and other resins15,16. Because of the strong reactivity between P-Cl bonds, hexachlorocyclotriphosphazene, or HCCP, is a flexible core molecule that readily undergoes nucleophilic substitution to generate cyclotriphosphazene derivatives.17,18,19. It is well known that cyclotriphosphazene, which consists of alternating nitrogen and phosphorus atoms, has high thermal stability20,21. Furthermore, the six-membered P-N ring of the substituted cyclotriphosphazene contributes to its chemical stability22,23. Interestingly, extensive studies using this molecule have led to its intended application in areas, such as flame retardants24,25. The remarkable properties of derivatives of cyclotriphosphazene have been used in the polymer industry to enhance the thermal and flame-retardant properties of polymers such as epoxy resin26,27. A cyclotriphosphazene-based epoxy synthesized by Seraji et al. was combined with diglycidyl ether of bisphenol F (DGEBF), an epoxy phenolic novolac resin28. A novel organic–inorganic hybrid polyphosphazene-modified manganese hypophosphite shuttle (PZS-MnHP) shuttle prepared to enhance the flame retardancy and anti-drip behavior of polyethylene terephthalate (PET). A comprehensive analysis of the flame-retardation procedure was also conducted29. A novel flame-retardant ingredient, hexachlorocylodiphosphazane, was physically added to epoxy varnish to provide both biocidal and flame-retardant properties. To evaluate any drawbacks associated with the additions, an analysis of the mechanical and physical resistances was also carried out30,31,32. Several arylhydrazone ligands and their Cu and Ni metal complexes have been discussed as potential flame retardant and antibacterial additives for polyurethane surface coatings33,34,35. A new additive based on a new sulphonamide ligand and its copper metal complex was added to a specific polyurethane varnish to make the coated film antibacterial and flame retardant36. In order to manufacture new flame-retardant paint, Cu(II) and Cd(II) metal complexes were prepared, and their efficacy was evaluated. The lowest possible oxygen percentage and ignitability were needed for combustion to continue37. Certain elements, such as nitrogen and boron, can combine with organophosphorus flame retardants to intensify their flame-retardant properties. Researchers have recently found that the synergistic effects of the chemical reaction between phosphorus and sulphur elements in polymer materials can rapidly increase the flame-retardant effects of polymer materials when organophosphorus flame retardants are prepared with sulfur elements in polymer polymerization38,39,40,41. The main objective of this study is the creation of novel substituted cyclodiphospha(V)zane derivatives to increase the exterior wood paint formulation flame-retardant properties. It was anticipated that this would make the material more resistant to ignition. Its flammability is also reduced with a limiting oxygen index (LOI) value. Furthermore, the effect of the type of alkyl chain length used on the mechanical and flame-retardant qualities is investigated.

High purity and analytical reagent grade (AR) chemicals were used in this study. Sulfadiazine, ortho nitro aniline, PCl5, copper chloride dihydrate (CuC12·2H2O), cadmium chloride dihydrate (CdC12·2H2O), diethyl ether, ethanol, and dimethyl sulfoxide (DMSO) were acquired from BDH Chemicals, UK. Merck products were used as indicators, including “perchloric acid, ammonia solution, ammonium chloride, nitric acid, ethylene-diaminetetraacetic acid disodium salt” (EDTA), Murexide, Eriochrome black-T (E.B.T), salicylic acid, and p-chloro-aniline.

After the complete degradation of the complexes, the metal contents were titrated against standard EDTA, and the metal concentration was calculated gravimetrically42. The phosphorus level was measured using phosphoammonium molybdate gravimetrically43. These micro-analytical determinations (C, H, N, Cl, and S) were performed at the Micro Analytical Center of Cairo University, Egypt. Using the KBr method, the IR spectra were recorded on a Perkin–Elmer 437 IR spectrophotometer (400–4000 cm–1). BurkerFTIR-400 MHZ spectrophotometer was used to measure the 1H and 13C NMR spectra, with TMS as the internal standard. A Sherwood Scientific Magnetic Susceptibility Balance” was used to record the solid-state magnetic susceptibilities of the complexes. Under a nitrogen environment, a “Shimadzu TGA-50H” with a flow rate of 20 mL min−1 was used for the thermogravimetric analysis. The UV–vis spectra were recorded using a “Perkin-Elmer Lambda 3B UV–vis spectrophotometer”, “Micro Analytical Centre, Cairo University.” Mass spectra were acquired using a direct “insertion probe” (DIP) and a “Shimadzu-Ge-Ms-QP 100-EX mass spectrometer” (Japan) within the temperature range of 50–1000 °C. Using MOE 2015 software, molecular docking studies were performed to examine the contacts and binding mechanism between the most active complexes and the proteins 5UHF and 4FM944. The obtained results were compared with glycerol (GOL) and N-(2-methylphenyl)-Nalpha-(selenophene-2-carbonyl)-d-phenylalanin-amide (88D), reference compounds, using molecular docking models. The protein data bank (http://www.rcsb.org.pdb) was used to obtain the crystal structures of human topoisomerase II alpha (PDB ID: 4FM9) and M. tuberculosis (PDB ID: 5UHF). The 3D structures of the ligands, the most potent complexes, 88D and GOL, which were stored as MDL mol files, were constructed using Chem Draws 18.0. The molecule with the lowest binding affinity value was given the highest score.

The ligand H2L was prepared using the techniques of Zhumurova and Kirzanov45 and Chapman et al.46, as mentioned in the lecture work47. For 30 min, a well-stirred cold solution of ligand H2L (4.7 g, 0.01 mol) in 100 mL acetonitrile in a quick-fit flask was supplemented with a small amount of [N′-2-pyridinylsulfaniliamide] (5 g, 0.02 mol) in 100 mL acetonitrile after finishing the addition. The reaction mixture was refluxed for about two hours. The reaction was completed, and the (HCl gas stopped evolving). The resulting solid product was separated by filtration and sanitized by washing several times with acetonitrile and diethyl ether, then dried under vacuum over anhydrous CaCl2 to give the corresponding substituted hexachlorocyclodiphosph(V)azane H2L ligand (Scheme 1) m.p = 205 °C; Brown yellow, 41% yield (4 g).

Synthesis of substituted cyclodiphospha(V)zane of the sulfa drug, H2L ligand.

After the successful preparation of the ligand, interest was focused on how it behaved with specific metal ions, including Cu(II) and Cd(II). The hot solution of H2L ligand (5 mmol) in 100 mL absolute ethanol was diluted dropwise with a hot solution of the aforementioned metal salts (10 mmol) in 50 mL absolute ethanol. The reaction mixture was refluxed for 2 h. The product underwent separation, then it was washed several times with ethanol and diethyl and then dried in vacuo.

The recently synthesized H2L ligand was subjected to elemental analyses, mass spectra, UV–vis, IR, and 1H, 13C NMR spectrum studies. The elemental analysis results (C, H, N, Cl, S, and P) accord well with the necessary values given by the suggested formulas. Scheme 1 and Table 1. The freshly synthesized ligand, H2L, exhibited a sharp melting point, indicating its purity. The molecular formula C32H26Cl4N12O8P2S2 is congruent with the mass spectrum, which revealed a base peak at m/z = 293.33 (100%), which is compatible with the molecular formula (C6H11CL13N3P2) and a molecular ion peak at m/z = 975.33 (12.82%), which is in good agreement with the proposed formula (Scheme 1). Figure 1. The UV–vis spectra of H2L-free ligand in 10–3 M with dimethylformamide (DMF) acting as a solvent. Table 2 displays the distinctive absorption band at 273 nm for the dimeric structure of the phosphorus four-membered ring51. The ligand spectrum displayed other absorption bands characteristic at 308 nm for π–π* transit transitions52.

Mass fragmentation of H2L-free ligand.

Table 3, Fig. 3 present infrared data for the H2L ligand revealed distinctive vibration bands at 1155 cm−1 and 660 cm−1, which correspond to the υ (P-N) and υ (P-Cl) groups, respectively21,53. Bands located at 3359 cm−1 and 1620 cm−1 are ascribed to υ(NH) and υ(C=N), respectively54. The distinct proton signals were visible in the 1H-NMR spectra of the isolated ligand H2L. Table 4 and Fig. 3, recorded in DMSO-d6 at room temperature. At 4.133, a wide signal was observed, indicating the (NH) proton of the enol/keto form of the H2L ligand, which can be exchanged with D2O51. On the other hand, a (–CH)hetro cyclic proton signal at δ = 8.48 ppm was proposed55. Furthermore, additional doublet-doublet signals for the H2L ligand were detected within the 6.59–7.99 ppm range attributable to aromatic protons56. The 13C NMR spectra of the H2L ligand revealed a peak at 40 ppm, which is attributable to DMSO-d6. The pyrimidine ringcarbon atom (C1) is assigned to the recorded signal at 115.84 and 116.07 ppm. Signals observed in the range (119.72–136.20) ppm are characteristic of different aromatic carbon positions, indicating distinct aromatic carbon locations57. The benzene carbon atoms attached to the nitrogen atoms of phospha(V)azo ring were assigned to the recorded signals at 146.62 and 149.06 ppm58. On the other hand, the remaining 13C signals at 157.57 and 158.76 ppm were due to (C=N) carbon atoms of the pyrimidine ring58, as seen in Table 5, Fig. 6.

In an attempt to clarify their molecular structures, new metal complexes of the H2L ligand of Cu(II) and Cd(II) were identified through elemental analyses (C, H, N, Cl, S, P, and M), molar conductance, electronic spectra Uv–vis, IR, 1H NMR, magnetic studies, thermal analyses TGA, and X-ray.

The elemental analyses (C, H, N, Cl, S, and P) of the isolated Cu(II) and Cd(II) metal complexes are in good agreement with those required by the proposed formulae of the complexes Fig. 2.

Suggested structures of Cu(II) and Cd(II) metal complexes.

Table 1 coincides fairly well with Fig. 2. The complexes are soluble in "DMSO" and "DMF" solvents but insoluble in water. The molar conductance (ΛM) studies of the complexes, which were performed at a concentration of 10−3 M using DMF as a solvent, show that the Cu(II) and Cd(II) complexes are not electrolytic51. The conductivity values were 2.02–1.74 Ω−1 cm2 mol−1, supporting their non-electrolytic behavior and neutrality of the coordination sphere59, as shown in Table 1. This finding confirms that the anions were inside the coordination sphere of the metal complex.

Table 2 displays the electronic spectra of the metal complexes in (10−3 M) DMF at room temperature in the 200–800 nm wavelength range. Complexes show a characteristic absorption band similar to the phosphazo four-member rings assigned of H2L ligand, which is shifted to lower wavelengths at 269 nm and 270 nm for [(CuCl2)2(H2L)(H2O)4] and [(CdCl2)2(H2L)(H2O)4] complexes respectively60,61. For metal complexes, the absorption bands linked to the π–π* transitions are moved to shorter wavelengths at 306 nm and 300 nm. Additionally, the spectra of Cu(II) and Cd(II) complexes show a band at 314 nm and at 346 nm, respectively, which is linked to the n–π* transition62. Furthermore, the Cu(II) complex spectra show a band at 334 nm and at 396 nm linked to the (L–MCT) ligand to metal charge transfer21,63. Two bands at 666 nm (15,015 cm−1) and 706 nm (14,164 cm−1) exhibited a d-d [2Eg → 2T2g(x2–y2) and (6A1g → 5T1g) transition for the Cu(II) complex, which suggested an octahedral environment64,65. Also, an octahedral environment for the copper complex was confirmed by the reported magnetic moment value of 1.5 B.M59. Moreover, an octahedral geometry is most likely postulated for the Cd(II) complex because d-d transitions are not expected for such a filled d10 system, and the observed bands for the Cd(II) complex are due to intra-ligand transitions and revealed a diamagnetic property66,67.

The infrared spectra of the freshly synthesized H2L ligand and those of its Cu(II) and Cd(II) complexes, shown in Table 3 and Fig. 3, were obtained in the range of 4000–400 cm−1 and assisted in identifying absorption regions caused by the corresponding vibrations. It demonstrates that the spectra of complexes displayed absorption bands at 682 cm−1, 684 cm−1, 1155 cm−1, and 1154 cm−1 are attributed to υ(P–Cl) and υ(P–N) for Cu(II) and Cd(II) complexes respectively53,64.

IR spectra of H2L-free ligand and its Cu(II) and Cd(II) complexes.

The IR spectra of the complexes were contrasted with those of the free ligand to identify the most probable coordination sites involved in chelation. Chelation was intended to have an impact on the location and intensity of these peaks. The sulfonamide group's υ(NH) was discovered at 3359 cm−1. According to a careful examination of the free ligand and its Cu(II) and Cd(II) metal complexes, they were hidden behind the wide bands, and new absorption broad bands of the enolic ʋ(OH) appeared at 3354 cm−1 and 3355 cm−1 in the spectrum of the isolated Cu(II) and Cd(II) complexes for the free ligand51. For the H2L ligand, the sulfone group in the double bond stretching region asym (O=S=O) and sym (O=S=O) were present at 1343 cm−1 and 1085 cm−1. Following interaction with Cu(II) and Cd(II), the transition metal ions in these sulfone group bands were relocated to lower frequencies at 1322 cm−1 and 1322 cm−1, respectively, than the ligand H2L. The red shift of the SO2 band to higher frequencies at 1092 cm−1 can be attributed to the change of the sulfonamide group (–SO2NH) to the enol form (–SO(OH)=N) as a result of complex formation to build a more stable six-membered ring64. The spectra also indicated a sharp band due to the stretched vibration rings of diazine (C=N), which occurred at 1620 cm−1 for the H2L and then was shifted to lower frequencies changed to 1585 cm−1 in the Cu(II) and Cd(II) Complexes spectra68. This finding suggests that the coordination via the pyrimidine-N atom of the heterocycle ring –N is involved in the production of complexes64. Coordinating water infrared bands (H2O) appeared at 841–843 cm−1 for metal complexes, indicating water molecules bonding to metal ions59. Thermal gravimetric analyses were used to demonstrate the coordinated nature of the water molecules. Additionally, there are two additional bands at 548–569 cm−1 and 410–546 cm−1 attributed to the ν(M–O) and ν(M–N) stretching vibrations, respectively, which are explained by the IR spectra of Cu(II)and Cd(II) metal complexes64. Form the above, it is evident that the H2L ligand coordinated with metal ions through two sites, the enolic OH of the sulfonamide group and pyrimidine-N.

Upon complexation using DMSOd6 as solvent, the 1H NMR spectra of Cd(II) complexes in Table 4 and compare the distinctive proton signals of the Cd(II) complex to those of the parent H2L ligand in Figs. 4, 5, 6. This shows new broad enolic(–SO(OH) = N–) signal at δ = 3.93–4.60 ppm was observed, which confirms the involvement of the sulfonamide group in the complexation69. Additionally, for diamagnetic complexes, the aromatic protons multi-signals first occurred at δ = 6.55–7.64 ppm62. For Cd(II) complexes, a new signal was discovered in the ring at δ = 3.17 ppm under complexation, corresponding to coordinated water molecules21,24.

1H NMR spectrum of H2L-free ligand.

1H NMR diagram of Cd(II) of H2L-free ligand.

13C NMR data of H2L-free ligand.

Water molecules are categorized as either coordinated within the inner coordination sphere or crystalline outside of it in Cu(II) and Cd(II) metal complexes, as determined by thermogravimetric analyses (TGA). These analyses also describe the thermal breakdown process of these chelates70. Table 6 provides a proof of the data, and the weight loss of each chelate was computed within the relevant temperature ranges. Within the temperature range of 38–1000 °C, TGA of the [(CuCl2)2(H2L)(H2O)4] complex of H2L-free ligand exhibits five stages of decomposition, as indicated by the TGA curves. The initial phase occurs between 38 and 137 °C (Fig. 7), resulting in the decomposition of 4H2O (Coordinated), with a measured mass loss of 5.80% (calcd. 5.47%). The second phase involves an approximate mass loss of 4.21% (calcd. 4.13%) with a temperature range of 137–224 °C loss of 2HCN. The third decomposition stage, occurring between 224 and 517 °C, is the loss of the organic part C10H8Cl4N4P2S2O4 with an estimated mass loss of 39.19% (calcd. 39.92%). The fourth stage, conducted at temperatures of 517–848 °C, involves the loss of organic compound C10H12Cl3N3O2, resulting in a mass loss of 24.92% (calcd. 23.71%). Finally, the fifth stage at the temperature 848–1000 °C resulted in the decomposition of the organic part C10H7N4 and HCl with a found mass loss of 16.61% (calcd. 16.6%), leaving a metallic residue of 2CuO. The total weight loss is 90.65% (calcd. 88.77%).

TGA cruve of Cu(II) complex.

Moreover, the TGA analysis curves of the [(CdCl2)2(H2L)(H2O)4] metal complex of H2L-free ligand revealed three decomposition stages occurring between 39 and 1000 °C Fig. 8. The first stage, beginning at 39–156 °C, involves the removal of four coordinated 4H2O molecules, causing mass loss of 5.22% (calcd 5.09%). The second stage occurred at 156–462 °C; there was a mass loss of 47.71% (calc. 47.83%) due to the removal of HCl and the organic parts C20H13Cl4N4O2P2S2. During the third stage, at a temperature of 462–1000 °C, a mass loss of 29.09% (calcd. 29.06%), resembling the elimination of the organic parts C12H13Cl4N8O4. The stable 2CdO metallic residue is the ultimate product. Meanwhile, the net weight loss was 81.97% (calcd. 82.91%).

TGA curve of Cd(II) complex.

According to these study results, we proposed a structure for the newly prepared ligand and its metal complex as presented in Scheme 1 and Fig. 2.

A powder X-ray diffractometer was used to determine the crystalline structure of the synthesized H2L-ligand and its [(CuCl2)2(H2L)(H2O)4]and[(CdCl2)2(H2L)(H2O)4] complexes. Figure 9 displays the powder XRD patterns of the ligand and metal complexes. Their sharp peaks indicated their crystalline structure36. Metal ions cause variances in X-ray patterns between the metal complexes, which are crystalline, and their original ligand. The average crystal size of the samples was calculated using the Debye–Scherrer equation. The formula D = K λ/βCosθ where β is the full-width half maximum (FWHM) of the characteristic peak, θ is the Bragg diffraction angle for the hkl plane, and λ is the wavelength of the used X-ray source, which is 1.54 Å, K is a constant equal to 0.9436,71. The average crystalline size of the free H2L-ligand Cu(II) and Cd(II) metal complexes was 9.7–5.6 and 8.7 nm, respectively.

X-ray powder diffractogram of the H2L-free ligand, Cu (II), and Cd(II) metal complexes.

Docking was performed on the M. tuberculosis receptor (PDB ID: 5UHF) using the proteins 5UHF with MOE 2015 software to assess the docking activity of the most potent produced compounds H2L, [(CdCl2)2(H2L) (H2O)4]), and the control (88D). The docked molecules had a higher binding affinity (− 6.10 kcal mol−1) than the control 88D, which is greater. The docking energy scores for the metal-complexes of and docked ligands were determined to be − 8.48, − 7.76, and − 9.25 kcal mol−1 for the (a) H2L and [(CdCl2)2(H2L) (H2O)4] complexes, respectively (Table 7). The negative binding energy increases in proportion to the intensity of the interaction. The interaction occurred in the order of H2L > [(CdCl2)2(H2L) (H2O)4]. These results are consistent with the examination of antibacterial efficacy. Figure 10 illustrates the bonding interactions between the docked molecules and 88, including hydrogen bonds and polar and hydrophobic interactions with particular amino acid residues in the 5UHF protein. The MOE 2015 software analyzed the most potent compounds, H2L and [(CdCl2)2(H2L)(H2O)4], together with the control GOL, concerning the human topoisomerase II alpha receptor docking (PDB ID: 4FM9). The docked molecules exhibit a higher binding affinity of − 3.76 kcal mol−1 when compared to the control GOL. The docking energy score for the metal complexes and docked ligands in the (a) H2L and [(CdCl2)2(H2L) (H2O)4] complexes was determined to be − 12.45 and − 9.13 kcal mol−1, respectively, as shown in Tables 7 and 8. The stronger the engagement, the higher the negative energy score. The interaction proceeded according to the pattern H2L > [(CdCl2)2(H2L) (H2O)4]. These conclusions align with the anticancer properties observed in the investigation. Figure 11 displays the total bonding interactions among the amino acid residues in the 4FM9 protein, the docked molecules, and GOL, encompassing hydrogen bonds and polar and hydrophobic interactions.

2D & 3D interaction of H2L and 88D in the active site of M. tuberculosis (PDB ID: 5UHF). Hydrogen bonds are displayed in cyan and H-pi-bonds in dark magenta.

2D & 3D interactions H2L,[(CdCl2)2(H2L)(H2O)4], and GOL in the active site of human topoisomerase II alpha (PDB ID: 4FM9). Hydrogen bonds are displayed in cyan.

The activity of ligands and their metal complexes as antibacterial and anticancer agents was demonstrated by docking them against PDB IDs 5UHF and 4FM9. The substances exhibit significant protein binding and numerous interactions.

Cyclodiphosph(V)azane ligand H2L and its Cu(II) and Cd(II) metal complexes were added to the formulation of wood paint at a ratio of 1.0% to prepare the coating compositions. Table 9 lists the varnish composition in tabular form. The samples with different molar ratios were then applied to the wood panels using a brush. Every attempt was made to keep the film thickness constant at 60 ± 5 µm to assess its mechanical and physical characteristics.

The preparation of the steel panels followed the ASTM D: 609 standards. Moreover, the dry film thickness was measured according to the ASTM D: 1005 standard. While film hardness was measured according to the ASTM D523 standard in measuring the gloss level. The ASTM D: 3363 standard was followed in measuring the film hardness. The adhesion and flexibility were tested using ASTM D: 3359 and ASTM D: 522, respectively, while the resistance to mechanical damage, or “impact resistance,” was determined using ASTM Method D: 2794-93.

The mechanical and physical characteristics of the coating were influenced by adding cyclodiphosph(V)azane and its Cu(II) and Cd(II)) complexes and these effects were evaluated following certain standards. This was conducted to determine if adding the chemicals had any adverse consequences. The examined attributes were adhesion, flexibility, cross hatch, scratch toughness, and gloss. Relevant outcomes were provided. The results in Table 10 indicate that adding metal complexes substantially enhanced gloss, cross-hatch adhesion, hardness, impact resistance values, and chemical resistance39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72.

Additionally, the prepared cyclodiphosph(V)azane derivative enhanced the mechanical properties of the dry-painted films, including the gloss levels, when used as the basis for the synthesized Cu(II) and Cd(II) complexes. Cross-hatch adhesion levels range from 4B to 5B, hardness by pencil (kg) (1.5–2.5), and impact (J) ranges from 1.3 to 2.2. Furthermore, the metal complexes identified can improve the accuracy of this test in comparison to the blank sample, which lacks manufactured additives like ligands and its Cu and Cd metal complexes. The favorable outcomes can be attributed to the complex structure that includes aromatic rings, P, N, Cu, and Cd, which improve the mechanical characteristics.

Time to ignition and heat release:

The TTI and THR are crucial parameters to consider when assessing the flammability and flame retardancy of interior materials73. This test was conducted in compliance with ASTM E17648,49. Table 11 demonstrates the TTI of coated plywood samples with their Cu(II) and Cd(II) metal complexes, both in their original state (blank sample) and after being treated with a phosphazene ligand. The findings revealed that the coated wood sample, treated with a wood paint formulation blended with phosphazene ligand and its Cu(II) and Cd(II) metal complexes, had the longest TTI compared to other samples, including uncoated plywood sample and coated samples mixed only with cyclophoshazene ligand. The inclusion of the cyclophosphazene ligand and its Cu(II) and Cd(II) metal complexes was observed to improve the TTI and THR. These improvements of incorporated paint formulation with either cyclophosphazene ligand or its Cu(II) and Cd(II) metal complexes may be attributed to (1) firstly according to the incorporation with the cyclophosphazene ligand, this is due to the presence of synthesized phosphorus, various elements of nitrogen and chlorine atoms, (2) the presence of Cu(II) and Cd(II) metal complexes beside the synthesized phosphorus, various elements of nitrogen, which form an excellent intumescent carbon layer with high mechanical characteristics and thermal stability. Moreover, results indicated that the heat flux increased from 10 to 25 kW m−2, causing all specimens to ignite with different ignition periods ranging from 550 to 1200 s, respectively. This suggests that the uncoated wood sample burns more readily. Furthermore, when compared to a coated plywood sample containing incorporated paint mixed with a manufactured phosphazene ligand and its Cu(II) and Cd(II) metal complexes to the burnt specimen, the latter would have shattered and emitted significant smoke due to the emission of CO and CO2.

Using rheometric scientific equipment, the minimum oxygen concentration in a mixture of nitrogen and oxygen needed to maintain specimen ignition was determined following the standard50.

The flame-retardant properties of the wood paint were assessed by using phosphazene derivatives and their Cu(II) and Cd(II) metal complexes through the LOI test. A higher oxygen concentration requirement indicates a more flame-resistant sample. The test findings demonstrated that involving phosphazene ligand and its Cu(II) and Cd(II) metal complexes in the wood paint formulation, as indicated in Table 11, enhanced the flame-retardant properties of the coating in comparison to a blank sample. Table 11 indicates that samples coated with wood paint mixed with cyclophosphazene ligand or its metal complexes had flame retardant times ranging from 90 to 130 s, respectively. In contrast, the blank and plywood-coated samples without these additives had LOI values of 21% and 26%, respectively. Due to the presence of about 21% oxygen in the atmosphere, a material with a lower LOI than 21% would readily combust in the air. A material with an LOI value between 21 and 28% is considered “slow-burning.” Additionally, a substance is considered self-extinguishing when the flame or ignition source is eliminated. The incorporation of the phosphazene ligand and its Cu(II) and Cd(II) metal complexes increased the LOI value of the blended wood paint system. The alkyd thermoset polymer network in the paint formulation comprises the phosphazene ligand along with Cu(II) and Cd(II) metal complexes to improve the paint flame-retardant properties. Due to the presence of nitrogen, chlorine, and phosphorus along with Cu(II) and Cd(II) in phosphazene compounds, they exhibit increased fire behavior compared to blank samples because of the same response presented previously (the presence of Cu(II) and Cd(II) metal complexes beside the synthesized phosphorus, various elements of nitrogen, which form an excellent intumescent carbon layer with high mechanical), thereby providing improved flame-retardant features. The burnt samples are all depicted in Image 3, which included the blank sample, the coated sample without generated ligand, and the coated sample with paint combined with the synthesized compounds.

Shows the burn of an uncoated plywood sample, coated without incorporation with prepared compounds, and coated based on prepared H2L-ligand its Cu(II) and Cd(II) metal complexes.

Mechanism using halogen (chlorin) in the synthesis of phosphazene derivatives.

The flame-retardant functions by absorbing heat as the material heats up, releasing denser than air hydrogen halide gas on the material surface to remove oxygen and generating active free radical HO during the combustion in the material's gas phase. The flame-retardant effect is achieved by reacting with hydrogen halide, forming stable and inert free radicals. Halogenated flame retardants disrupt the chain reaction by preventing hydroxide and hydrogen free radicals from interacting with oxygen and carbon monoxide. Combustion is impeded by the trapping of radicals, which interrupts the chemical reaction of the exothermic oxidative flame. Here are the mechanisms of the reactions in the vapor phase.

H. + HX = H2 + X

HO.+ HX = H2O + X

RH + X.= R. + HX

Mechanism by using phosphorus in the prepared phosphazene derivatives.

Organophosphorus flame retardants typically operate through the following mechanism: (1) When the system of flame-retardant crashes, volatile radicals (PO, PO2, etc.) are released into the gas phase. These radicals can trap active radicals (H and HO) and stop the combustion chain reaction. The breakdown of organophosphorus flame retardants will simultaneously produce a non-combustible gas diluted into the surrounding air, reducing the oxygen level. (2) The breakdown and condensation of organophosphorus flame retardants in the condensed phase facilitates the matrix dehydration and carbonization to form a carbon layer. Moreover, it can develop a glass-like molten coating on the combustion surface upon thermal decomposition, which further inhibits heat and gas transmission, resulting in a flame-retardant effect. Combining certain elements, such as boron, nitrogen, etc., with organophosphorus flame retardants might enhance their flame-retardant attributes. Recent studies suggest that the flame-retardant properties of polymer materials can be rapidly increased by creating organophosphorus flame retardants that include sulfur-based components. The synergistic effect results from the interaction between the phosphorus and sulfur components in the polymer substance. Thus, organophosphorus compounds, such as phosphazene, are mostly employed in intumescent paints and coatings. In addition, foams that undergo expansion are created by intumescent systems. Because of this property, they safeguard flammable materials like wood and plastic and materials such as steel that degrade under high temperatures.

Mechanism using nitrogen-based flame retardants through the synthesized phosphazene derivatives.

Nitrogen-based flame retardants are particularly interesting due to their low toxicity, less smoke emission in flames, and excellent flame-retardant efficiency. Nitrogen flame-retardants produce non-flammable gases during combustion that can impede gas phase combustion and generate a flame-retardant effect. Reduce oxygen and gaseous fuel levels by generating NH3 or N2 while heating, allowing them to diffuse gradually into the combustion zone. Nevertheless, flame retardants containing nitrogen have a restricted amount of nitrogen. The material lacks both compatibility and flame-retardant efficacy.

The cyclotriphosphazene-based chemicals would enable further investigation into the flame retardant of steel, wood, and textiles. This is because flame retardant coating applications and cyclotriphosphazene-based flame retardants have excellent chelating performance.

The potential applications of our synthesized compounds beyond wood paint can be promising as anticorrosive materials for steel protection when incorporated with primer coating formulation in the future. The effectiveness of cyclotriphosphazene and its metal complexes encourages future corrosion inhibitors, along with anticorrosive coating applications, to have excellent with epoxy resins, and with different polymers which would be important in the study of corrosion inhibition. Also, the prepared compounds can be used as insecticide agents and antimicrobial additives materials for protective coating,77,78,79,80.

The synthesized ligand H2L and its metal complexes were separated and characterized using chemical analysis, mass spectra, conductivity measurements, electronic spectrum data UV–vis, FT-IR, 1H,13C-NMR, TGA, XRD, and docking investigations. The prepared cyclophosphazene derivatives and their Cu(II) and Cd(II) metal complexes were incorporated into the paint formulations for coating plywood samples. Further research was carried out to assess the flame-retardant properties of the coated samples using ignition time and LOI tests. The synthesized chemicals included in the experiment enhanced the maximum LOI value and improved the flame-retardant quality of the coated formulation. When combined, nitrogen and other elements can boost the flame-retardant attributes of organophosphorus flame retardants. Organophosphorus flame retardants can be synthesized by leveraging the synergistic effects of the chemical reaction between phosphorus and Sulfur elements in the polymer materials. This process rapidly enhances the flame-retardant properties of polymer materials.

The datasets used and analyzed during the current study are available from the corresponding author upon reasonable request.

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Chemistry Department, Faculty of Science (Girls), Al-Azhar University, Nasr City, Cairo, Egypt

Narmeen G. El khashab, Salwa A. H. Albohy & Carmen M. Sharaby

Chemistry Department, Faculty of Science (Boys), Al-Azhar University, Nasr City, Cairo, Egypt

H. Abd El-Wahab

Pre-Treatment and Finishing of Cellulose Based Textiles, Textile Research and Technology Institute (TRT), National Research Center, 33-El-Buhouth St, Dokki, Cairo, Egypt

Moustafa M. G. Fouda

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all authors contribued equally for this manuscript

Correspondence to H. Abd El-Wahab.

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El khashab, N.G., Albohy, S.A.H., El-Wahab, H.A. et al. Preparation and characterization of novel flame-retardant paint of substituted cyclodiphosph(V)azane sulfonomide and their Cu(II), Cd(II) metal complexes as new additives for exterior wood coating protection. Sci Rep 14, 14452 (2024). https://doi.org/10.1038/s41598-024-64065-w

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Received: 26 March 2024

Accepted: 05 June 2024

Published: 24 June 2024

DOI: https://doi.org/10.1038/s41598-024-64065-w

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