# Advancements in Flame-Retardant Systems for Rigid Polyurethane Foam
###### Abstract  
The amplified employment of rigid polyurethane foam (RPUF) has accentuated the importance of its flame-retardant properties in stimulating demand. Thus, a compelling research report is essential to scrutinize the recent progression in the field of the flame retardancy and smoke toxicity reduction of RPUF. This comprehensive analysis delves into the conventional and innovative trends in flame-retardant (FR) systems, comprising reactive-type FRs, additive-type FRs, inorganic nanoparticles, and protective coatings for flame resistance, and summarizes their impacts on the thermal stability, mechanical properties, and smoke toxicity suppression of the resultant foams. Nevertheless, there are still several challenges that require attention, such as the migration of additives, the insufficient interfacial compatibility between flame-retardant polyols or flame retardants and the RPUF matrix, and the complexity of achieving both flame retardancy and mechanical properties simultaneously. Moreover, future research should focus on utilizing functionalized precursors and developing biodegradable RPUF to promote sustainability and to expand the applications of polyurethane foam. rigid polyurethane foam; flame retardancy; smoke toxicity suppression; flame retardants 2  
human health [18]. For this reason, it is crucial to explore flame-retardant RPUF and to expand its practical applications [5; 19; 20; 21; 22; 23]. There are three primary approaches being utilized to address the limitations of RPUF, as depicted in Table 1. The first method is copolymerization, which entails the chemical modification of the surface and core of the polymer [24; 25]. The final two methods involve adding flame-retardant (FR) additives to the polymer matrix through mixing or coating [5; 21; 26; 27; 28; 29]. Meanwhile, the first option has been demonstrated to be the durable and feasible approach for pristine RPUF. These flame retardants are primarily composed of organic compounds which possess a flame-retardant segment that can create covalent bonds with RPUF [30]. As a result, the integration of flame retardants into RPUF composites can lead to a considerable improvement in the compatibility between the polymer and the flame retardants as well as provide additional benefits such as enhanced mechanical properties, improved thermal stability, and better compatibility with other additives [31; 32]. Numerous reviews have been published regarding the utilization of additive-type flame retardants to enhance the flame resistance of RPUF, and this has been established as the most practical and economical approach for untreated RPUF [28; 33; 34]. Phosphorus-based and nitrogen-based flame retardants are among the most frequently utilized additives in the fabrication of RPUF composites [35]. Phosphorus-based and nitrogen-based FRs are the most used FRs in RPUF composites. Phosphorus-based FRs, such as ammonium polyphosphate (APP) and melamine polyphosphate (MPP), have excellent flame-retardant properties and generate char upon exposure to heat, which further protects the polymer matrix from combustion [33; 36]. Nitrogen-based FRs, such as melamine cyanurate (MC) and guanidine phosphate (GP), generate nonfllammable gases and effectively suppress the flames during combustion. Furthermore, the use of nanocomposites that contain nanofillers, like layered double hydroxides (LDHs) and graphene oxide (GO), has demonstrated potential as effective flame retardants and smoke suppressants for RPUF composites [37; 38]. These nanofillers have a large surface-area-to-volume ratio, improving their ability to interact with the polymer matrix and impede the spread of flames. The growing utilization of RPUF has intensified the significance of its flame-retardant properties in driving demand. Consequently, an imperative research report is warranted to investigate recent advancements in the field of the flame retardancy and smoke toxicity suppression of RPUF, given the limited extant literature on this topic.