Synthesis and application of poly (cyclotriphosphazene‐resveratrol) microspheres for enhancing flame retardancy of poly (ethylene terephthalate)

Synchronous modification of ZIF-67 with cyclomatrix polyphosphazene coating for efficient flame retardancy and mechanical reinforcement of epoxy resin

Cyclomatrix polyphosphazenes have attracted widespread attention in the field of polymer flame retardancy. Nevertheless, the optimal manifestation of their distinctive structural attributes and flame-retardant properties necessitates a judicious selection of condensation monomers and synergistic templates during the fabrication of polyphosphazene flame retardants. In our previous studies, it was discovered that when ZIF-67 is functionalized with polyphosphazene, the by-product HCl from phosphazene polycondensation causes etching on ZIF-67. Based on this "synchronous etching" effect, a series of hybrid materials comprising cyclomatrix polyphosphazene and ZIF-67, denoted as ZIF-67@PDS (PDS, poly-(cyclotriphosphazene-co-4,4'-diaminodiphenyl sulfone)), ZIF-67@PBS (PBS, poly-(cyclotriphosphazene-co-Bisphenol A)), and ZIF-67@PZS (PZS, poly-(cyclotriphosphazene-co-4,4'-sulfonyldiphenol)), was synthesized utilizing DDS (4,4'-diaminodiphenyl sulfone), BPA (Bisphenol A), and BPS (4,4'-sulfonyldiphenol) monomers as precursors, respectively. Upon the incorporation of 2.0 wt.% of ZIF-67@PDS, ZIF-67@PBS, and ZIF-67@PZS, the flame retardant and mechanical characteristics of EP composites exhibited marked enhancement. The unique structural characteristics of hybrid and the synergistic effects of Co-P-N contribute to the improvement of comprehensive properties. Compared with pure EP, EP/ZIF-67@PZS has the best enhancement effect, and its pHRR, THR, and TSP decreased by 34.0%, 30.0%, and 40.5%, respectively. In terms of mechanical strength, ZIF-67@PZS also increases the flexural strength of EP by 37.42%. Relying on the "synchronous etching" effect, this study explores and verifies the effective combination of ZIF-67 and different types of polyphosphazenes, and obtains a series of ZIF-67-derived cyclomatrix polyphosphazene hybrids with different morphologies and properties in one step. It provides a new idea and strategy for the simultaneous modification of polyphosphazene materials and the preparation of multifunctional flame retardants in the future.

Rapid Crystallization and Fluorescence of Poly(ethylene terephthalate) Using Graphene Quantum Dots as Nucleating Agents

In this study, graphene quantum dots (GQDs) with a diameter of ~3 nm were successfully synthesized and incorporated into a poly(ethylene terephthalate) (PET) matrix to fabricate PET/GQDs nanocomposites. The impact of GQDs on the crystallization and thermal stability of the PET/GQDs nanocomposites was investigated. It was observed that the addition of only 0.5 wt% GQDs into the nanocomposites resulted in a significant increase in the crystallization temperature (peak temperature) of PET, from 194.3 °C to 206.0 °C during the cooling scan process. This suggested that an optimal concentration of GQDs could function as a nucleating agent and effectively enhance the crystallization temperature of PET. The isothermal crystallization method was employed to analyze the crystallization kinetics of the PET/GQDs nanocomposites, and the data showed that 0.5 wt% GQDs significantly accelerated the crystallization rate. Furthermore, the incorporation of GQDs into the PET matrix imparted photoluminescent properties to the resulting PET/GQDs nanocomposites. The PET crystals with GQDs as crystal nuclei and the crazes caused by defects played a vital role in isolating and suppressing the concentration quenching of GQDs. This effect facilitated the detection of defects in PET.

Synthesis of DOPO-Based Phosphorus-Nitrogen Containing Hyperbranched Flame Retardant and Its Effective Application for Poly(ethylene terephthalate) via Synergistic Effect

To obtain industrialized poly(ethylene terephthalate) (PET) composites with highly efficient flame retardancy, a phosphorus-nitrogen (P-N) containing hyperbranched flame retardant additive was synthesized by 9,10-dihydro-9-oxa-10-phospho-phenanthrene-butyric acid (DDP) and tris(2-hydroxyethyl) isocyanurate (THEIC) through high temperature esterification known as hyperbranched DDP-THEIC (hbDT). The chemical structure of the synthesized hbDT was determined by FTIR, ¹H NMR, ¹³C NMR, and GPC, etc. Subsequently, hbDT/PET composites were prepared by co-blending, and the effects of hbDT on the thermal stability, flame retardancy, combustion performance, and thermal degradation behavior of PET were explored to deeply analyze its flame retardant mechanism. The test results showed that hbDT was successfully synthesized, and that hbDT maintained thermal stability well with the required processing conditions of PET as retardant additives. The flame retardant efficiency of PET was clearly improved by the addition of hbDT via the synergistic flame-retardant effect of P and N elements. When the mass fraction of flame retardant was 5%, the LOI of the hbDT/PET composite increased to 30.2%, and the vertical combustion grade reached UL-94 V-0. Compared with pure PET, great decreased total heat release (decreased by 16.3%) and peak heat release rate (decreased by 54.9%) were exhibited. Finally, the flame retardant mechanism of hbDT/PET was supposed, and it was confirmed that retardant effect happened in both the gas phase and condensed phase. This study is expected to provide a new idea for the development of low toxic, environment-friendly and highly efficient flame retardant additive for polyesters in an industry scale.

Fire retardant polyethylene terephthalate containing 4,4′-(hexafluoroisopropylidene)diphenol-substituted cyclotriphosphazene microspheres

Polyphosphazene derivatives are gaining popularity due to their eco-friendly character and high content of flame-retardant components. Herein, a polyphosphazene derivative (PZAF) microsphere was successfully synthesized utilizing an in-situ template approach, which was then employed as an additive flame retardant in polyethylene terephthalate (PET) to improve the fire safety. Thermogravimetric analysis revealed that PZAF promoted the pyrolysis of PET in advance to generate a stable char layer that protects the matrix from heat, consequently increasing char residues. With addition of 10 wt% PZAF, the PET nanocomposites obtained a V-0 grade in vertical combustion test and its LOI value increased from 24.2 vol% to 32.1 vol%. Moreover, the peak heat release and carbon monoxide production decreased by 46.6% and 50.6%, respectively. This was because the phosphonic acid fragments and pyridine ring compounds produced by the PZAF pyrolysis encouraged the development of a robust char layer. Meanwhile, the •PO radicals generated by the pyrolysis of PZAF could capture free radicals in the gas phase, ultimately ending the chain reaction of combustion. Also, mechanical properties of the PET nanocomposites were noticeably enhanced by the addition of 3 or 5 wt% PZAF.

Morphology-Controlled Synthesis of Polyphosphazene-Based Micro- and Nano-Materials and Their Application as Flame Retardants

Common flame retardants, such as halogen-based materials, are being phased-out owing to their harmful environmental and health effects. We prepared poly-(cyclotriphosphazene-co-4,4′-sulfonyldiphenol) (PZS) microspheres, nanotubes, capsicum-like nanotubes, and branched nanotubes as flame retardants. An increase in reaction temperature changed the morphology from nanotubes to microspheres. A PZS shape had a positive effect on the flame retardancy of polyethylene terephthalate (PET). The PZS with a capsicum-like nanotube morphology had the best flame retardancy, and the PET limiting oxygen index increased from 25.2% to 34.4%. The flame retardancy capability was followed by PZS microspheres (33.1%), branched nanotubes (32.8%), and nanotubes (32.5%). The capsicum-like nanotubes promote the formation of highly dense and continuous carbon layers, and they release a non-combustible gas (CO2). This study confirms polyphosphazene-based flame retardants as viable and environmentally-friendly alternatives to common flame retardants. It also presents a novel and facile design and synthesis of morphology-controlled nanomaterials with enhanced flame retardant properties.