A Bridge-Linked Phosphorus-Containing Flame Retardant for Endowing Vinyl Ester Resin with Low Fire Hazard

The high flammability of vinyl ester resin (VE) significantly limits its widespread application in the fields of electronics and aerospace. A new phosphorus-based flame retardant 6,6'-(1-phenylethane-1,2 diyl) bis (dibenzo[c,e][1,2]oxaphosphinine 6-oxide) (PBDOO), was synthesized using 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and acetophenone. The synthesized PBDOO was further incorporated with VE to form the VE/PBDOO composites, which displayed an improved flame retardancy with higher thermal stability. The structure of PBDOO was investigated using Fourier transformed infrared spectrometry (FTIR) and nuclear magnetic resonances (NMR). The thermal stability and flame retardancy of VE/PBDOO composites were investigated by thermogravimetric analysis (TGA), vertical burn test (UL-94), limiting oxygen index (LOI), and cone calorimetry. The impacts of PBDOO weight percentage (wt%) on the flame-retardant properties of the formed VE/PBDOO composites were also examined. When applying 15 wt% PBDOO, the formed VE composites can meet the UL-94 V-0 rating with a high LOI value of 31.5%. The peak heat release rate (PHRR) and the total heat release (THR) of VE loaded 15 wt% of PBDOO decreased by 76.71% and 40.63%, respectively, compared with that of untreated VE. In addition, the flame-retardant mechanism of PBDOO was proposed by analyzing pyrolysis behavior and residual carbon of VE/PBDOO composites. This work is expected to provide an efficient method to enhance the fire safety of VE.

Barrier Effects of Cellulosic Fibers with Hybrid Coating Based on Zirconium Metal-Organic Framework

Metal-organic frameworks (MOFs) have great potential for the development of fire barriers for flammable materials. Accordingly, zirconium-based metal-organic framework (Zr-MOF), branched polyethyleneimine (BPEI), and vinyltriethoxysilane (VTES) were deposited to produce composites assembled on cellulosic fibers to investigate their barrier effects. The structure, morphology, and thermal properties of the cellulosic fibers were characterized using FTIR spectroscopy, SEM, and TGA. Compared with the untreated cotton sample, the temperature of the maximum rate of weight loss (T_max) of C-Zr-MOF/BPEI/VTES increased from 479 to 523.3 °C and the maximum weight loss rate (R_max) at T_max decreased from 37.6 to 17.2 wt%/min. At 800 °C, the pristine cotton was burned out without residues whereas the residual char content of the C-Zr-MOF/BPEI/VTES sample was 7.2355 wt%. From the vertical burning tests, the results suggested that the C-Zr-MOF/BPEI/VTES sample had better barrier effects by reducing the flame-spread speed and generating more protective char layers.

Flame-Retardant Performance of Transparent and Tensile-Strength-Enhanced Epoxy Resins

In this study, a flame-retardant additive with 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) groups denoted DSD was successfully synthesized from DOPO, 4,4′-diaminodiphenyl sulfone (DDS), and salicylaldehyde. The chemical structure of DSD was characterized by FTIR–ATR, NMR, and elemental analysis. DSD was used as an amine curing agent, and the transparent, tensile strength-enhanced epoxy resins named EP–DSD were prepared via thermal curing reactions among the diglycidyl ether of bisphenol A (DGEBA), 4,4′-diaminodiphenylmethane (DDM), and DSD. The flame-retardancy of composites was studied by the limiting oxygen index (LOI) and UL-94 test. The LOI values of EP–DSD composites increased from 30.7% for a content of 3 wt % to 35.4% for a content of 9 wt %. When the content of DSD reached 6 wt %, a V-0 rating under the UL-94 vertical test was achieved. SEM photographs of char residues after the UL-94 test indicate that an intumescent and tight char layer with a porous structure inside was formed. The TGA results revealed that EP–DSD thermosets decomposed ahead of time. The graphitization degree of the residual chars was also investigated by laser Raman spectroscopy. The measurement of tensile strength at breaking point shows that the loading of DSD increases the tensile strength of epoxy thermosets. Py-GC/MS analysis shows the presence of phosphorus fragments released during EP–DSD thermal decomposition, which could act as free radical inhibitors in the gas phase. Owing to the promotion of the formation of intumescent and compact char residues in the condensed phase and nonflammable phosphorus fragments formed from the decomposition of DOPO groups, EP–DSD composites displayed obvious flame-retardancy.