A Systematic Review and Bibliometric Analysis of Flame-Retardant Rigid Polyurethane Foam from 1963 to 2021

Flame-retardant science and technology are sciences developed to prevent the occurrence of fire, meet the needs of social safety production, and protect people's lives and property. Rigid polyurethane (PU) is a polymer formed by the additional polymerization reaction of a molecule with two or more isocyanate functional groups with a polyol containing two or more reactive hydroxyl groups under a suitable catalyst and in an appropriate ratio. Rigid polyurethane foam (RPUF) is a foam-like material with a large contact area with oxygen when burning, resulting in rapid combustion. At the same time, RPUF produces a lot of toxic gases when burning and endangers human health. Improving the flame-retardant properties of RPUF is an important theme in flame-retardant science and technology. This review discusses the development of flame-retardant RPUF through the lens of bibliometrics. A total of 194 articles are analyzed, spanning from 1963 to 2021. We describe the development and focus of this theme at different stages. The various directions of this theme are discussed through keyword co-occurrence and clustering analysis. Finally, we provide reasonable perspectives about the future research direction of this theme based on the bibliometric results.

Influence of Titanium Dioxide Modified Expandable Graphite and Ammonium Polyphosphate on Combustion Behavior and Physicomechanical Properties of Rigid Polyurethane Foam

In this research, the individual influence and synergistic behavior between titanium dioxide modified expendable graphite and ammonium polyphosphate on combustion behavior and physicomechanical properties of rigid polyurethane foam (RPUF) were investigated. Combustion behavior was evaluated by limiting oxygen index, and vertical-combustion tests. Thermal stability was studied via thermogravimetric/differential thermal gravimetric (TG/DTG) analysis. Results showed that the modified expendable graphite presented better thermal stability and flame retardancy for RPUF than the normal expandable graphite. Furthermore, the combination of the modified expendable graphite and ammonium polyphosphate with the mass ratio of 1 : 1 caused the RPUF to exhibit better flame retardancy, compression strength and high temperature thermal stability. Especially, the compression strength of this polymer composite sharply increased by 52.4 % over RPUF.

Effect of Particle Size of Additives on the Flammability and Mechanical Properties of Intumescent Flame Retarded Polypropylene Compounds

The effect of particle size reduction of the components of a common intumescent flame retardant system, consisting of pentaerythritol (PER) and ammonium polyphosphate (APP) in a weight ratio of 1 to 2, was investigated on the flammability and mechanical performance of flame retarded polypropylene (PP) compounds. Additives of reduced particle size were obtained by ball milling. In the case of PER, the significant reduction of particle size resulted in inferior flame retardant and mechanical performance, while the systems containing milled APP noticeably outperformed the reference intumescent system containing as-received additives. The beneficial effect of the particle size reduction of APP is explained by the better distribution of the particles in the polymer matrix and by the modified degradation mechanism which results in the formation of an effectively protecting carbonaceous foam accompanied with improved mechanical resistance. Nevertheless, 10% higher tensile strength was measured for the flame retarded PP compound when as-received APP was substituted by milled APP.

Flame-Retardant and Thermal Degradation Mechanism of Caged Phosphate Charring Agent with Melamine Pyrophosphate for Polypropylene

An efficient caged phosphate charring agent named PEPA was synthesized and combined with melamine pyrophosphate (MPP) to flame-retard polypropylene (PP). The effects of MPP/PEPA on the flame retardancy and thermal degradation of PP were investigated by limiting oxygen index (LOI), vertical burning test (UL-94), cone calorimetric test (CCT), and thermogravimetric analysis (TGA). It was found that PEPA showed an outstanding synergistic effect with MPP in flame retardant PP. When the content of PEPA was 13.3 wt% and MPP was 6.7 wt%, the LOI value of the flame retardant PP was 33.0% and the UL-94 test was classed as a V-0 rating. Meanwhile, the peak heat release rate (PHRR), average heat release rate (AV-HRR), and average mass loss rate (AV-MLR) of the mixture were significantly reduced. The flame-retardant and thermal degradation mechanism of MPP/PEPA was investigated by TGA, Fourier transform infrared spectroscopy (FTIR), TG-FTIR, and scanning electron microscopy-energy dispersive X-ray spectrometry (SEM-EDXS). It revealed that MPP/PEPA could generate the triazine oligomer and phosphorus-containing compound radicals which changed the thermal degradation behavior of PP. Meanwhile, a compact and thermostable intumescent char was formed and covered on the matrix surface to prevent PP from degrading and burning.