Surface Flame-Retardant Systems of Rigid Polyurethane Foams: An Overview

Abstract

Rigid polyurethane foam (RPUF) is one of the best thermal insulation materials available, but its flammability makes it a potential fire hazard. Due to its porous nature, the large specific surface area is the key factor for easy ignition and rapid fires spread when exposed to heat sources. The burning process of RPUF mainly takes place on the surface. Therefore, if a flame-retardant coating can be formed on the surface of RPUF, it can effectively reduce or stop the flame propagation on the surface of RPUF, further improving the fire safety. Compared with the bulk flame retardant of RPUF, the flame-retardant coating on its surface has a higher efficiency in improving fire safety. This paper aims to review the preparations, properties, and working mechanisms of RPUF surface flame-retardant systems. Flame-retardant coatings are divided into non-intumescent flame-retardant coatings (NIFRCs) and intumescent flame-retardant coatings (IFRCs), depending on whether the flame-retardant coating expands when heated. After discussion, the development trends for surface flame-retardant systems are considered to be high-performance, biological, biomimetic, multifunctional flame-retardant coatings.

Keywords: fire safety; flame retardant; rigid polyurethane foam; surface coating.

Figure 1

(a) Schematic diagram of the synthesis route of the PAAm–PDA hydrogel, (b) SEM image of the surface of the hydrogel-coated RPUF, (c) SEM image of the cross-sectional structure of the hydrogel-coated RPUF, (d) video screenshot of the burning process of uncoated and hydrogel-coated RPUF under the propane flame for 10 s, (e) HRR and (f) TSP curves of uncoated and hydrogel-coated RPUF, (g) and (h) are schematic of the flame-retardant mechanisms of the hydrogel-coated RPUF [29].

Figure 2

(a) Schematic diagram of preparation route of m-MXene nanosheets; (b) UV curing process of coated RPUF, (c) TTI, pHRR, and THR data of samples in cone test; (d) video screenshot of pure RPUF and coated RPUF samples during UL-94 testing [30].

Figure 3

(a) The histogram of LOI values of pure RPUF and treated RPUF samples, (b) THR, and (c) TSP curves of pure RPUF and treated RPUF samples; (d) video screenshot of pure RPUF, RPUF coated with BPR, and RPUF coated with BPR/Si-sol during UL-94 test; (e) schematic diagram of the flame-retardant mechanism of treated RPUF samples; (f) the data of p-HRR, THR, SPR, TSP, residual mass in cone test, and UL-94 test of pure RPUF and treated RPUF samples [31].

Figure 4

(a) Preparation route of SiO₂/RPUF composites, (b) SEM image of SiO₂/RPUF, (c) digital images of pure and SiO₂ aerogel-coated RPUF, (d) temperature vs. time curves of the upper surface points in thermal conductivity test, (e) HRR in cone test of pure PUF and SiO₂ aerogel-coated RPUF, and (f) specific density of neat and SiO₂ aerogel-coated RPUF in smoke chamber test [32].

Figure 5

(a) The schematic diagram of the SA manufacturing process; (b) the preparation route of SA-coated RPUF, (c) HRR, (d) SPR, (e) CO, and (f) CO₂ curves in the cone test of pure RPUF and SA-coated RPUF [47].

Figure 6

SEM micrograph of (a) RPUF/aerogel interface; Char residue of (b) RPUF and (c) aerogel after LOI test; (d) TGA curves of pure RPUF and alginate/clay aerogels with different ratios; (e) HRR, and (f) TSR curves of samples in cone test [50].

Figure 7

(a) The scheme of the manufacturing process of FRPU@PVH/BN/GP; (b) porous structure of char layer; (c) LOI value of neat RPUF and coated RPUF; (d) HRR, (e) TSR, and (f) CO curves in cone calorimetry test [36].

Figure 8

(a) Surface SEM images of pure RPUF and the coated RPUF samples, (b) optical photos of samples in UL-94 testing, and (c) flame-retardant mechanism schematic diagram of the RPUF with Si/EG coating [33].

Figure 9

(a) Cone results of pure RPUF and coated RPUF ((a₁). HRR curves; (a₂). THR curves; (a₃). TSP curves; (a₄). Mass loss curves); (b) digital photos of the char residue after cone tests ((b₁). RPUF0, (b₂). RPUF1, (b₃). RPUF2, (b₄). RPUF3) [34].

Figure 10

(a) Illustration of the SFR-RPU foam fabrication, (b) compression strength curves of pure RPU and RPU/P1B7M2, (c) photographs of pure RPU and RPU/P1B7M2 after UL-94 testing, (d) SEM of the interface where the RPU foam contacts with the coating, (e) cross-section and (f) surface of RPU/P1B7M2 in UL-94 test [57], (g) fabrication route of m-MXene nanosheets, (h) mechanical bar graphs of RPU, RPU/PBM, and RPU/PBM-m1.0 [58].

Figure 11

(a) SEM micrograph of the cross-sectional SPB/starch mixture coated RPUF after heating for 12 min in the pre-mixed flame. (b) Backside temperature versus time curves of the different coatings. (c) Water contact angles and elution ratios of the coatings (SPB mixed with different saccharides) on the surface of RPUF [65]. (d) The appearance of the coated RPUF (top: SPB/HEC; bottom: SPB/gellan gum) after sustaining 12 min of heating in the pre-mixed flame. (e) SEM micrograph of the cross-sectional carbon residue of SPB/HEC mixture-coated RPUF. (f) SEM micrograph of the cross-sectional carbon residue of SPB/gellan gum mixture-coated RPUF [66].

Figure 12

(a) The schematic diagram of the manufacturing process of PU-ATP. (b) Digital images of pure PU and PU-ATP (with 30% load) during the direct burning test. (c) HRR of pure PU and PU-ATP (with 30% load). Digital images before and after the cone test of (d) pure PU and (e) PU-ATP (with 30% load) [67].

Figure 13

Illustration of the SFR/RPUF modification.

Figure 14

(a) Schematic diagram of the synthetic route of poly (VS-co-HEA); (b) the phase-separated micro/nanostructure of poly (VS-co-HEA); (c) digital photo of poly (VS-co-HEA) coatings versus PU foam following shear testing, during which bulk PU foam broke before interfaces; (d) HRR, (e) TSR, and (f) mass loss curves in UL-94 test; (g) LOI of RPUF [69].

Figure 15

(a) IR spectra of hybrid flame-retardant coating on FRPU foam surface. Due to surface hydrophobic treatment, a strong peak at ~1200 cm⁻¹ can be observed, which is assigned to C-F; Digital and corresponding contact angles photos (inset) of FRPU samples (b) before and (c) after hydrophobic treatment [72].

Figure 16

SEM images of (a) the untreated wood, (b) MXene-coated wood, and (c) waterborne acrylic resin (WA) on MXene-coated wood. Water contact angle after 1 s of (d) natural wood, (e) MXene-coated wood, and (f) waterborne acrylic resin (WA) on MXene-coated wood [73].

Figure 17

Images of (a) GO and (b) G₁H_0.50 paper’s flame detection methods. (c) A schematic representation of the fire alarm sensor based on GO/HCPA paper operating in a high-temperature or flame attack scenario. Electrical resistance transition behavior of the G₁H_0.50 paper under (d) flame attacks and (e) different ambient temperatures [72].

Figure 18

(a) Schematic diagram of the preparation of FRPU@GO/CNTs@BN foam. (b) SEM photographs of the FRPU@GO/CNTs@BN cross-section morphology at various magnifications. (c) Flame detection of FRPU@GO/CNTs@BN. (d) The apparatus for sensing resistance variation with temperature. (e) The FRPU@GO/CNTs@BN electrical resistance varies in real-time as a function of temperature. (f) Coated PU foams’ electrical resistance variations over at 250 °C. (g) LOI and UL-94 test results of PU and coated PU foams [74].

Figure 19

(a) Diagram of the preparation of the MXene sheets and the corresponding SEM images; (b) video screenshot of the burning process of natural wood and M/wood at various times. (c) The sheet resistivity of wood and MXene loading at various MXene concentrations. (d) The sheet resistivity of wood and the cycles of spray coating at various MXene concentrations; the M/wood electrical contact for LED lighting bulbs are seen in the picture [73].