# Advancements in Flame-Retardant Systems for Rigid Polyurethane Foam
###### Abstract
This comprehensive review centers on conventional and emerging trends in flame-retardant (FR) systems, including reactive-type and additive-type FRs, inorganic nanoparticles, and protective coatings, aimed at enhancing the flame retardancy and smoke toxicity suppression of RPUF. This review also covers an overview of the preparation and properties of RPUF and the current trends in flame-retardant strategies for RPUF and a discussion on the future outlook of flame-retardant RPUF. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Flame-Retardant Type** & **Composite** & **Remarks** & **Ref.** \\ \hline \multirow{6}{*}{Reactive flame retardants} & - & The incorporation of 10.5 wt.\% & \\  & RPUF/AMPO & AMPO resulted in an increase in the & \\  & (polyol-bis(hydroxymethyl)-N, & LOI value from 20.0\% in the pure & \\  & N-bis(2-hydroxyethyl) & RPUF to 23.4\%. & \\  & aminomethylphosphine oxide) & - & However, the inclusion of AMPO in & \\  & & RPUF resulted in a 21.2\% decrease & \\  & & in compressive strength. & \\ \hline \hline \end{tabular}
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Table 1: Examples of RPUF composites with various flame retardants. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Flame-Retardant Type** & **Composite** & **Remarks** & **Ref.** \\ \hline  & & - & The incorporation of 8.0 wt.\% \\  & & glycerol/ethanol phosphate (GEP) & \\  & & resulted in an increase in the LOI of \\  & RPUF/GEP (glycerol/ethanol & RPUF to 23.5\% \\  & phosphate) & - & However, the total heat release \\  & & (THR) and peak smoke production \\ Reactive flame & & rate (PSPR) were increased by \\  & retardants & 52.0\% and 170.0\%. \\ \hline  & & - & By incorporating 10.0 wt.\% of the \\  & & additive, RPUF demonstrated an \\  & RPUF/PPGE (phenylphosphoryl glycol & LOI of 24.5\%, obtained a UL-94 V-1 \\  & ether oligomer) & rating, and experienced a notable \\  & & 1.5\% enhancement in \\  & & compressive strength. \\ \hline  & & - & The RPUF containing 10 wt.\% of \\  & & flame-retardant particles exhibited \\  & RPUF/pEG-P(MA) (pulverized & excellent flame retardancy, as \\  & expandable graphite & evidenced by a high LOI of \\  & (pEG)-poly(methyl & \\  & methacrylate-acrylic acid) copolymer) & - & The RPUF/pEG-P(MA) composites \\  & & demonstrated a compressive \\  & & modulus of 48.4 MPa and a \\  & & compressive strength of 2.8 MPa. \\ \hline  & & - & The incorporation of 15.0 wt.\% of \\  & & MATMP in RPUF resulted in a \\  & & UL-94 V-0 rating accompanied by a \\  & & 34.0\% decrease in PRHR and an \\ Additive flame & RPUF/MATMP (melamine amino & LOI of 25.5\%. \\  & trimethylene phosphate) & - & At lower loading levels \\  & & (<10.0 wt.\%), MATMP \\  & & simultaneously improved the \\  & & compressive strength and thermal \\  & & insulating properties of RPUF. \\ \hline  & & - & RPUF/MFAPP30 attained a V-1 \\  & & rating in the UL-94 test, exhibiting \\  & & an LOI of 21.3 vol.\% compared to \\  & RPUF/MFAPP & RPUF/APP30 with an equal load \\  & (melamine–formaldehyde & of APP. \\  & resin-microencapsulated ammonium & - & The RPUF/MFAPP30 \\  & polyphosphate) & demonstrated a compressive \\  & & strength of 0.295 MPa, exhibiting a \\  & & 13.5\% increase compared to \\  & & RPUF/APP30. \\ \hline  & & - & The RPUF coated with \\  & & alginate/clay aerogel exhibited an \\  & & impressive LOI of 60.0\%, with \\  & & 32.0\% and 37.0\% reductions in \\  & & PRHR and TSR as compared to the \\  & & untreated foam. \\  & & - & However, the presence of the \\  & & aerogel within the porous foam \\  & & noticeably compromised its thermal \\  & & conductivity. \\ \hline \hline \end{tabular}
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Table 1: _Cont._