Fire-Retardant Property of Hexasubstituted Cyclotriphosphazene Derivatives with Schiff Base Linking Unit Applied as an Additives in Polyurethane Coating for Wood Fabrication

Hexakis[p-(hydroxymethyl)phenoxy]cyclotriphosphazene as an Environmentally Friendly Modifier for Polyurethane Powder Coatings with Increased Thermal Stability and Corrosion Resistance

Protection against fire and the corrosion of metals is necessary to ensure human safety. Most of the fire and corrosion inhibitors do not meet the ecological requirements. Therefore, effective and ecological methods of protecting metals are currently a challenge for researchers. In this work, the influence of hexakis(4-(hydroxymethyl)phenoxy)cyclotriphosphazene (HHPCP) on the characteristics of powder coatings was examined. The coatings' properties were investigated by measuring the roughness, hardness, adhesion to the steel surface, cupping, gloss, scratch resistance, and water contact angle. The thermal stability was studied by furnace test and TGA analysis. The corrosion resistance test was carried out in a 3.5% NaCl solution. The distribution of phosphazene-derived segments in the coating was examined by GD-EOS analysis. Modified coatings show better corrosion and thermal resistance and can be used for the protection of the steel surface. Their better corrosion resistance is due to the electroactive properties of the phosphazene ring and its higher concentration at the coating surface, confirmed by GD-EOS analysis. The increase in thermal resistance is due to the effect of the formation of phosphoric metaphosphoric and polyphosphoric acids during the decomposition of HHCPC, which remain in the condensed char phase and play a crucial role in surface protection.

The Effect of Alkyl Terminal Chain Length of Schiff-Based Cyclotriphosphazene Derivatives towards Epoxy Resins on Flame Retardancy and Mechanical Properties

A series of Schiff-based cyclotriphosphazenes with different alkyl chain length terminal ends, 4a (dodecyl) and 4b (tetradecyl), were synthesized and the structures were characterized using Fourier-transform infrared spectroscopy (FT-IR), and 1H, 13C, and 31P nuclear magnetic resonance (NMR) and carbon, hydrogen, and nitrogen (CHN) elemental analysis. The flame-retardant and mechanical properties of the epoxy resin (EP) matrix were examined. The limiting oxygen index (LOI) of 4a (26.55%) and 4b (26.71%) revealed a good increment compared to pure EP (22.75%). The LOI results corresponded to their thermal behavior studied using thermogravimetric analysis (TGA) and the char residue analyzed under field emission scanning electron microscopy (FESEM). The mechanical properties of EP showed a positive impact on tensile strength with a trend of EP < 4a < 4b. The tensile strength went from 8.06 N/mm2 (pure EP) to 14.36 and 20.37 N/mm2, indicating that the additives were compatible with epoxy resin.

Fire Retardancy and Dielectric Strength of Cyclotriphosphazene Compounds with Schiff Base and Ester Linking Units Attached to the Electron-Withdrawing Side Arm

A series of compounds with Schiff base and ester linking units attached to the electron-withdrawing side arm (Cl, NO₂, and OH) have been successfully synthesized through four schemes of the chemical route. These compounds were characterized using Fourier Transform Infrared spectroscopy (FTIR), Nuclear Magnetic Resonance spectroscopy (NMR), and Carbon, Hydrogen and Nitrogen (CHN) elemental analysis. The epoxy resin was used as a matrix of molding to observe the refinement of fire-retardant properties of the modified cyclotriphosphazene compounds. The fire-retardant testing was done using Limiting Oxygen Index (LOI). The LOI value of pure epoxy resin was increased from 22.75% to 24.71% when incorporated with 1 wt.% of hexasubstituted cyclotriphosphazene (HCCP). Interestingly, all the final compounds gave a positive increment in the LOI value and the highest LOI value was obtained from the compound containing a nitro side arm with LOI value of 26.90%. In order to understand the thermal stability of these compounds, Thermogravimetric Analysis (TGA) was carried out. The compound with the nitro group at the terminal end has the highest char residue which is 34.2% at 700 °C. This indicated that the presence of the nitro withdrawing group was able to enhance the fire retardancy of the materials. Based on SEM observation, the shape of the final compound’s char residue demonstrated the formation of a porous protective layer with a dense surface. The dielectric property was conducted according to ASTM D149 AC breakdown voltage to determine its dielectric strength. The results showed that the highest dielectric strength value belonged to the compound containing a nitro group side arm with 24.41 kV/mm⁻¹ due to the π electron delocalization.