Simultaneously Improved Flame Retardance and Ceramifiable Properties of Polymer-Based Composites via the Formed Crystalline Phase at High Temperature

Preparation of a Ceramifiable Phenolic Foam and Its Ceramization Behavior

Ceramifiable phenolic foam (GC-PF) with a low ceramization temperature has been prepared by incorporation of low melting point glass frits (LMG) containing B₂O₃ and Na₂O as main components into a phenolic resin matrix. Fourier transform infrared spectrometry, X-ray diffractometry, and scanning electron microscopy were used for assessment of the structure, phase composition, and morphology of GC-PF before and after combustion analysis, respectively. A glassy ceramic protective layer is formed when GC-PF is exposed to flame or a high temperature environment. The presence of LMG not only reduces the level of defects in the phenolic foam cell wall (gas escape pore), but also promotes the generation of a glassy ceramic protective layer that could inhibit heat feedback from the combustion zone and reduce the rate of formation of volatile fuel fragments. Thermogravimetric analysis and differential scanning calorimetry were used to establish that GC-PF exhibits excellent thermal stability. Limiting oxygen index (LOI) determination suggests that GC-PF displays good flame retardancy. The LOI of GC-PF was as high as 45.6%, and the char residue at 900 °C was six times greater than that for ordinary phenolic foam (O-PF). The area of the raw material matrix of GC-PF after combustion for 60 s was about 1.7 times larger than that for O-PF. A possible mode of formation of glassy ceramics has been proposed.

Raman and ab initio analyses of ion pairs in concentrated K[B(OH)4] droplets

In this study, microscopic Raman spectroscopy and Ab initio quantum chemical calculation were used to determine the structural details of ion pairs and their transformation in concentrated K[B(OH)₄] droplets. The Raman experiment shows that the v_sym-B(OH)₄^- undergoes a downward shift with the decrease of WSR. The contact ion pairs (CIPs) change to solvent shared ion pairs when the molar water-to-solute ratio (WSR) is bigger than 6; CIPs are the dominant species when 1.33 < WSR < 6, where K⁺ bonds to [B(OH)₄^-] in bidentate form (CIP-II); the CIPs quickly dehydrate and associate to triple ion pairs (TIPs) when WSR < 5. Raman experiment and ab initio quantum chemical calculation show that TIPs are mainly present in "anionic" type such as {[B(OH)₄^-]K⁺[B(OH)₄^-](H₂O)_n}, where K⁺ bonds to two [B(OH)₄^-] in bidentate or/and tridentate form (TIP-a-II or/and TIP-a-III). When WSR <1.33, most TIPs convert to complex clusters such as chain-like structure. The remaining TIPs associate to six-membered ring structure [B₃O₃(OH)₄^-] and the relative content increases from 0 to 20% when the WSR ranges from 1.33 to 0.55.

Ammonium Polyphosphate with High Specific Surface Area by Assembling Zeolite Imidazole Framework in EVA Resin: Significant Mechanical Properties, Migration Resistance, and Flame Retardancy

A zeolite imidazole framework (ZIF-67) was assembled onto the surface of ammonium polyphosphate (APP) for preparing a series multifunctional flame-retardant APP-ZIFs. The assembly mechanism, chemical structure, chemical compositions, morphology, and specific surface area of APP-ZIFs were characterized. The typical APPZ1 and APPZ4 were selected as intumescent flame retardants with dipentaerythritol (DPER) because of their superior unit catalytic efficiency of cobalt by thermogravimetric analysis. APPZ1 and APPZ4 possessed 6.8 and 92.1 times the specific surface area of untreated APP, which could significantly enhance the interfacial interaction, mechanical properties, and migration resistance when using in ethylene-vinyl acetate (EVA). With 25% loading, 25% APPZ4/DPER achieved a limiting oxygen index value of 29.4% and a UL 94 V-0 rating, whereas 25% APP/DPER achieved a limiting oxygen index value of only 26.2% and a V-2 rating, respectively. The peak of the heat release rate, smoke production rate, and CO production rate respectively decreased by 34.7%, 39.0%, and 40.1%, while the char residue increased by 91.7%. These significant improvements were attributed to the catalytic graphitization by nano cobalt phosphate and the formation of a more protective char barrier comprised of graphite-like carbon.

Nanoreinforcements of Two-Dimensional Nanomaterials for Flame Retardant Polymeric Composites: An Overview

Polymer materials are ubiquitous in daily life. While polymers are often convenient and helpful, their properties often obscure the fire hazards they may pose. Therefore, it is of great significance in terms of safety to study the flame retardant properties of polymers while still maintaining their optimal performance. Current literature shows that although traditional flame retardants can satisfy the requirements of polymer flame retardancy, due to increases in product requirements in industry, including requirements for durability, mechanical properties, and environmental friendliness, it is imperative to develop a new generation of flame retardants. In recent years, the preparation of modified two-dimensional nanomaterials as flame retardants has attracted wide attention in the field. Due to their unique layered structures, two-dimensional nanomaterials can generally improve the mechanical properties of polymers via uniform dispersion, and they can form effective physical barriers in a matrix to improve the thermal stability of polymers. For polymer applications in specialized fields, different two-dimensional nanomaterials have potential conductivity, high thermal conductivity, catalytic activity, and antiultraviolet abilities, which can meet the flame retardant requirements of polymers and allow their use in specific applications. In this review, the current research status of two-dimensional nanomaterials as flame retardants is discussed, as well as a mechanism of how they can be applied for reducing the flammability of polymers.

Confined Dispersion of Zinc Hydroxystannate Nanoparticles into Layered Bimetallic Hydroxide Nanocapsules and Its Application in Flame-Retardant Epoxy Nanocomposites

In many fields, nanoparticles are frequently dispersed onto kinds of nanocarriers integrated into hybrid nanocomposites to acquire advanced performance. However, the nanoparticles usually tend to agglomerate on the surface, according to traditional synthetic methods. Besides, the exposed state of loaded nanoparticles and the weak adhesion with the supporters make them fall off during practical application, leading to "second agglomeration" of the nanoparticles and attenuated synergistic effects. In this work, we engineered layered bimetallic (Ni-Co) hydroxides (NCHs) into enclosed nanocages derived from metal organic frameworks (MOFs). Zinc hydroxystannate (ZHS) nanoparticles were selected to be confined dispersed within the hollow cavity of the three-dimensional nanocages. ZHS nanoparticles were tightly immobilized, monodispersing to form a novel multiyolk@shell nanostructure with NCH nanocages. To prove the effectiveness of this structural design, the as-synthesized hybrids ZHS@NCH were introduced into the epoxy matrix to inquiry its performance. Compared to neat ZHS, neat NCH, and physical mixture of ZHS and NCH, ZHS@NCH conferred better flame retardancy, thermal stability, and mechanical properties upon the epoxy nanocomposites. With the adding amount of 6 wt % ZHS@NCH, the UL-94 rating of the nanocomposite was V-0, and the peak of heat release rate value was reduced by 69.1%, while the mechanical properties were slightly influenced. The ingenious synthetic strategy gives insights into uniform distribution of nanoparticles within nanocapsules and enlightens the facile fabrication of multiyolk@shell nanomaterials.