Rigid polyurethane foams with halogen‐free flame retardants: Thermal insulation, mechanical, and flame retardant properties

Enhanced Fire Safety of Energy-Saving Foam by Self-Cleavage CO2 Pre-Combustion and Phosphorus Release Post-Combustion

Rigid polyurethane foam (RPUF) is widely utilized in construction and rail transportation due to its lightweight properties and low thermal conductivity, contributing to energy conservation and emission reduction. However, the inherent flammability of RPUF presents significant challenges. Delaying the time to ignition and preventing flame spread post-combustion is crucial for ensuring sufficient evacuation time in the event of a fire. Based on this principle, this study explores the efficacy of using potassium salts as a catalyst to promote the self-cleavage of RPUF, generating substantial amounts of CO2, thereby reducing the local oxygen concentration and delaying ignition. Additionally, the inclusion of a reactive flame retardant (DFD) facilitates the release of phosphorus-oxygen free radicals during combustion, disrupting the combustion chain reaction and thus mitigating flame propagation. Moreover, potassium salt-induced catalytic carbonization and phosphorus derivative cross-linking enhance the condensed phase flame retardancy. Consequently, the combined application of potassium salts and DFD increases the limiting oxygen index (LOI) and reduces both peak heat release rate (PHRR) and total heat release (THR). Importantly, the incorporation of these additives does not compromise the compressive strength or thermal insulation performance of RPUF. This integrated approach offers a new and effective strategy for the development of flame retardant RPUF.

Lightweight Biobased Polyurethane Composites Derived from Liquefied Polyol Reinforced by Biomass Sources with High Mechanical Property and Enhanced Fire-Resistance Performance

Lightweight biobased insulation polyurethane (BPU) composite foams with high fire-resistance efficiency are interested in building effective energy and low environmental impact today. This study focuses on manufacturing lightweight BPU from liquefied bamboo polyols and biomass resources, including rice husk and wood flour. Then, they are combined with three flame retardant (FR) additives, such as aluminum diethyl phosphinate, aluminum trihydroxide, and diammonium phosphate, to improve their fire resistance performance. The physicochemical properties, microstructure, thermal stability, mechanical properties, and flame-retardant properties of the BPU composites are characterized to optimize their compromise properties. The results showed that composites with optimized FRs achieved UL94 V-0 and those with nonoptimized FRs reached UL94 HB. The limiting oxygen index exhibited that the fire resistance of BPU composites could increase up to 21-37% within FR additives. In addition, the thermal stability of BPU composites was significantly improved in a temperature range of 300-700 °C and the compressive strength of the BPU composites was also enhanced with the presence of FRs. The scanning electron microscopy observation showed an influence of FRs on the morphology and cell size of the BPU composites. The bio-PU-derived samples in this study showed significantly low thermal conductivity values, demonstrating their remarkable thermal insulation effectiveness.

Thermal Stability and Flame Retardancy of Rigid Polyurethane Foam Composites Filled with Phase-Change Microcapsule

The flammability of rigid polyurethane foam (RPUF) limits its application. A new type of chitosan phase-change microcapsule (CS/PCM) was successfully prepared by the condensation method with chitosan and gum acacia as the wall material and paraffin as the core material. CS/PCM was introduced into RPUF composite material as filler to improve the thermal and flame-retardant properties of polyurethane. The morphology, structure, thermal properties and flame retardancy of the materials were characterized by scanning electron microscopy (SEM), X-ray diffractometer (XRD), thermogravimetric (TG) analysis, differential scanning calorimetry (DSC) and cone calorimetry. It is found that when the CS/PCM content is 30 wt%, the latent heat of phase transition of RPUF-30 is 12.308 J/g, the limiting oxygen index (LOI) is 26.1% and the fire risk is reduced. The flame-retardant mechanism shows that the barrier effect provided by chitosan plays an important role in effectively blocking the transfer of heat and combustible gas, and improving the flame-retardant property of the composite. This paper provides a new idea for the application of CS/PCM in RPUF.

Advancements in Flame-Retardant Systems for Rigid Polyurethane Foam

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.

Porous Thermal Insulation Polyurethane Foam Materials

Porous thermal insulation materials (PTIMs) are a class of materials characterized by low thermal conductivity, low bulk density and high porosity. The low thermal conductivity of the gas enclosed in their pores allows them to achieve efficient thermal insulation, and are they among the most widely used and effective materials in thermal insulation material systems. Among the PTIMs, polyurethane foam (PUF) stands out as particularly promising. Its appeal comes from its multiple beneficial features, such as low density, low thermal conductivity and superior mechanical properties. Such attributes have propelled its broad application across domains encompassing construction, heterogeneous chemical equipment, water conservation and hydropower, and the aviation and aerospace fields. First, this article outlines the structure and properties of porous thermal insulation PUF materials. Next, it explores the methods of preparing porous thermal insulation PUF materials, evaluating the associated advantages and disadvantages of each technique. Following this, the mechanical properties, thermal conductivity, thermal stability, and flame-retardant characteristics of porous thermal insulation PUF materials are characterized. Lastly, the article provides insight into the prospective development trends pertaining to porous thermal insulation PUF materials.

Methods to Increase or Decrease Resistance to Photodegradation and Biodegradation of Polyurethane/Polyisocyanurate (PU/PIR) Foams

Two series of rigid polyurethane-polyisocyanurate (PU/PIR) foams were obtained. They were modified using powder fillers, such as industrial food cocoa (K5-K15 foam) and instant freeze-dried coffee (KR-KR15) added in amounts of 5, 10 and 15 wt.%. W foam (reference) was obtained without filler. The foams were degraded in a climate chamber for 1 week, 2 weeks or 3 weeks. Appropriate temperature, humidity and UV radiation were set in the chamber, which did not change throughout the degradation process. The foams were also degraded in an oven for two days at 120 °C. The foam tests carried out indicated, among others, on the decrease in compressive strength along with the increase in the residence time of the samples in the chamber. Degraded foams also changed color. Foams containing 5% and 10% of industrial cocoa or freeze-dried coffee were more susceptible to degradation. The addition of 15% coffee or cocoa slows down the degradation process. In the present study, industrial food cocoa and instant freeze-dried coffee were used as modifiers of rigid PU/PIR foam. These fillers have two functions: they accelerate the biodegradation of foams and have antioxidant properties.

Role of hemp fiber addition on thermal stability, heat insulation, air permeability and cellular structural features of rigid polyurethane foam

In this current study, rigid polyurethane foams (RPUFs) composites were prepared using different percentage (3, 6, 9 and 12%) of the hemp fibers via one-shut one-step polymerization method. The influences of the hemp fiber addition on the RPUFs were investigated meticulously by means of Fourier-transform infrared spectroscopy (FTIR), thermo gravimetric analysis (TGA), thermal conductivity measurement and scanning electron microscopy (SEM) techniques by evaluating the alternations in the chemical structures of the component, thermal stability, apparent density, insulation performance and cellular topology of the produced samples. The structural analysis revealed that there existed the strong secondary chemical bonds between the functional groups belonging to the components and, depending on that, the improvement in the thermal stability of the foam samples was recorded accompanied by the formation of the better interfacial adhesion. Furthermore, thermal conductivity values of the hemp fiber-loaded RPUFs were observed to increase regularly with the increasing of the content level of the hemp fibers. This was explained by enhancement in the bulk phase conduction level depending of the apparent density rising, reduction in CO2 concentration inside cells as well as the formation of the distorted cellular structures. The obtained air permeability results displayed that the hemp fibers incorporated successfully with RPUF structure, which provides the occurrence of the novel micro barriers and pathways limiting the passage of the air throughout the matrix. The taken scanning electron microscopy images also indicated that the cellular morphology and dimensional stability of the produced foams affected negatively by the hemp fiber addition. At high contents, the wrinkled, non-uniform and irregular cellular structures were observed with ruptured and collapsed walls and struts.

Flame-Retarded Rigid Polyurethane Foam Composites with the Incorporation of Steel Slag/Dimelamine Pyrophosphate System: A New Strategy for Utilizing Metallurgical Solid Waste

Rigid polyurethane (RPUF) was widely used in external wall insulation materials due to its good thermal insulation performance. In this study, a series of RPUF and RPUF-R composites were prepared using steel slag (SS) and dimelamine pyrophosphate (DMPY) as flame retardants. The RPUF composites were characterized by thermogravimetric (TG), limiting oxygen index (LOI), cone calorimetry (CCT), and thermogravimetric infrared coupling (TG-FTIR). The results showed that the LOI of the RPUF-R composites with DMPY/SS loading all reached the combustible material level (22.0 vol%~27.0 vol%) and passed UL-94 V0. RPUF-3 with DMPY/SS system loading exhibited the lowest pHRR and THR values of 134.9 kW/m² and 16.16 MJ/m², which were 54.5% and 42.7% lower than those of unmodified RPUF, respectively. Additionally, PO· and PO₂· free radicals produced by pyrolysis of DMPY could capture high energy free radicals, such as H·, O·, and OH·, produced by degradation of RPUF matrix, effectively blocking the free radical chain reaction of composite materials. The metal oxides in SS reacted with the polymetaphosphoric acid produced by the pyrolysis of DMPY in combustion. It covered the surface of the carbon layer, significantly insulating heat and mass transport in the combustion area, endowing RPUF composites with excellent fire performance. This work not only provides a novel strategy for the fabrication of high-performance RPUF composites, but also elucidates a method of utilizing metallurgical solid waste.

Development of Rigid Polyurethane Foams Based on Kraft Lignin Polyol Obtained by Oxyalkylation Using Propylene Carbonate

This study aimed to develop new rigid polyurethane foams (RPUFs) for thermal insulation based on kraft lignin, the main by-product of the pulp and paper industry. Crude lignin-based polyol (LBP) was obtained via the oxyalkylation of kraft lignin using propylene carbonate (PC). A design of experiments (DoE) was used to evaluate the effect of the isocyanate (NCO)-to-hydroxyl (OH)-group’s ratio, the content of crude LBP, the blowing agent (BA), and catalyst on the thermal conductivity and density of RPUFs. Statistical analysis revealed that the increase in crude LBP and BA content in the formulation decreases the thermal conductivity and density of the foams. In addition, the fact that LBP is a viscous polyol containing PC-oligomers appears to affect the cellular structure of RPUFs, and consequently reduces their mechanical and thermal properties. The main novelty of this study consisted in the careful optimization of the formulation, namely, with regard to the type of blowing agent and with the high content of crude LBP obtained from the oxyalkylation of LignoBoost kraft lignin without purification to obtain good quality RPUF that meets market requirements for insulation materials.

Emerging trends in flame retardancy of rigid polyurethane foam and its composites: A review

Owing to the superior thermal insulating attributes of rigid polyurethane foam (RPUF) compared to other insulating materials (expanded and extruded polystyrene, mineral wool), it remains the most dominant insulating material and most studied polymer foam. Like other polyurethane foam, RPUF is highly flammable, necessitating the incorporation of flame retardants (FR) during production to lower combustibility, promoting its continuous use as insulation material in construction, transportation, and others. The popular approaches for correcting the high flammability of RPUF are copolymerization and blending (with FR). The second method has proven to be most effective as there are limited trade-offs in RPUF properties. Meanwhile, the high flammability of RPUF is still a significant hindrance in emerging applications (sensors, space travel, and others), and this has continuously inspired research in the flame retardancy of RPUF. In this study, properties, and preparation methods of RPUF are described, factors responsible for the high flammability of PUF are discussed, and flame retardancy of RPUF is thoroughly reviewed. Notably, most FR for RPUF are inorganic nanoparticles, lignin, intumescent FR systems of expandable graphite (EG), ammonium polyphosphate (APP), and hybridized APP or EG with other FR. These could be due to their ease of processing, low cost, and being environmentally benign. Elaborate discussion on RPUF FR mechanisms were also highlighted. Lastly, a summary and future perspectives in fireproofing RPUF are provided, which could inspire the design of new FR for RPUF.

Investigation of the relation between free volume and physico-mechanical performance in rigid polyurethane foam containing turkey feather fibers: Part 2

Rigid polyurethane foams (RPUFs) were modified with 0–15 wt.% turkey feather fibers (TFFs) produced from waste turkey feathers. One-shut free rising method was used for the production of TFFs-filled-RPUFs in a closed mold. The dependence of mechanical performance and water vapor permeability (WVP) feature of the final foams on TFFs loading was evaluated with free volume change. The free volume analysis was performed via Positron Annihilation Lifetime Spectroscopy (PALS), while the mechanical and WVP characteristics were determined with the use of the universal tester machines. PALS findings showed that the incorporation of TFFs with RPUF matrix caused the considerable diminishment in the free volume due to TFFs serving as a filling material and formation of strong secondary bonds between components. Moreover, tensile strength and extension of the foams decreased with the increasing of TFFs, which caused by the occurrence of noteworthy restriction on the spatial alignment and orientation capability of polyurethane chains due to the lack of sufficient free volume allowing the chains to move freely. As for the compression tests, all the TFFs-loaded RPUFs depicted substantially lower performance due to TFFs interfering with the ordered organization of isocyanate domains. Moreover, impact test results showed that the addition of TFFs into RPUF matrix brought about the insufficient impact energy delocalization throughout the matrix due to the restriction on the mobility of polymer chains. Additionally, the remarkable diminishment in WVP was recorded due to the reduction in the number of vacancies and constitution of keratin composed of roundly 60% of hydrophilic protein (especially cystine). All in all, this study established a strong links between free volume and characteristics of TFFs-loaded RPUFs.

Thermal Insulating Rigid Polyurethane Foams with Bio-Polyol from Rapeseed Oil Modified by Phosphorus Additive and Reactive Flame Retardants

In this article, rigid polyurethane foams obtained with the addition of a bio-polyol from rapeseed oil, were modified with the dimethyl propane phosphonate as additive flame retardant and two reactive flame retardants diethyl (hydroxymethyl)phosphonate and diethyl bis-(2-hydroxyethyl)-aminomethylphosphonate. The influence of used flame retardants on the foaming process and characteristic processing times of tested polyurethane systems were determined. The obtained foams were tested in terms of cell structure, physical and mechanical properties, as well as flammability. Modified foams had worse mechanical and thermal insulation properties, caused by lower cellular density and higher anisotropy coefficient in the cross-section parallel to the foam rise direction, compared to unmodified foam. However, the thermal conductivity of all tested foam materials was lower than 25.82 mW/m∙K. The applied modifiers effectively reduced the flammability of rigid polyurethane foams, among others, increasing the oxygen index above 21.4 vol.%, reducing the total heat released by about 41-51% and the rate of heat release by about 2-52%. A correlation between the limiting oxygen index values and both total heat released parameters from the pyrolysis combustion flow calorimetry and cone calorimetry was observed. The correlation was also visible between the value of the heat release capacity (HRC) parameter obtained from the pyrolysis combustion flow calorimetry and the maximum average rate of heat emission (MARHE) from the cone calorimeter test.

A Review of Rigid Polymeric Cellular Foams and Their Greener Tannin-Based Alternatives

This review focuses on the description of the main processes and materials used for the formulation of rigid polymer foams. Polyurethanes and their derivatives, as well as phenolic systems, are described, and their main components, foaming routes, end of life, and recycling are considered. Due to environmental concerns and the need to find bio-based alternatives for these products, special attention is given to a recent class of polymeric foams: tannin-based foams. In addition to their formulation and foaming procedures, their main structural, thermal, mechanical, and fire resistance properties are described in detail, with emphasis on their advanced applications and recycling routes. These systems have been shown to possess very interesting properties that allow them to be considered as potential substitutes for non-renewable rigid polymeric cellular foams.

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.

It Takes Two to Tango: Synergistic Expandable Graphite–Phosphorus Flame Retardant Combinations in Polyurethane Foams

Due to the high flammability and smoke toxicity of polyurethane foams (PUFs) during burning, distinct efficient combinations of flame retardants are demanded to improve the fire safety of PUFs in practical applications. This feature article focuses on one of the most impressive halogen-free combinations in PUFs: expandable graphite (EG) and phosphorus-based flame retardants (P-FRs). The synergistic effect of EG and P-FRs mainly superimposes the two modes of action, charring and maintaining a thermally insulating residue morphology, to bring effective flame retardancy to PUFs. Specific interactions between EG and P-FRs, including the agglutination of the fire residue consisting of expanded-graphite worms, yields an outstanding synergistic effect, making this approach the latest champion to fulfill the demanding requirements for flame-retarded PUFs. Current and future topics such as the increasing use of renewable feedstock are also discussed in this article.

Effect of perlite on the thermal conductivity, thermal degradation and mechanical properties of epoxy materials

In this study, the thermal conductivity, thermal degradation and mechanical properties of the perlite added epoxy materials were investigated. Perlite was added into the epoxy resin (EP) at the rate of 2%, 4%, 6%, 8% and 10% compared to the total mass. For the analysis of these epoxy materials, the thermal conductivity, density, hardness, X-ray diffraction (XRD), FTIR spectra, thermogravimetric analysis (TGA), tensile strength, flexural strength, limit oxygen index (LOI) and vertical burning tests were carried out. With the added perlite (PER10), a decrease of 24.03% occurred in the thermal conductivity coefficient compared to the pure EP material. A maximum decrease of 6.86% was detected in its density. It gained fire retardant feature as it increased from 6.15% to 28.71% in V-0 and carbon residue as inflammability class. Despite the decrease of tensile stress, tensile modulus increased. The maximum increase PER02 was 36.9%. It did not provide any improvement in terms of other mechanical properties.

Characterization and Properties of Water-Blown Rigid Polyurethane Foams Reinforced with Silane-Modified Nanosepiolites Functionalized with Graphite

In the present study, a promising flame retardant consisting of 80 wt% silane-modified nanosepiolites functionalized with 20 wt% graphite (SFG) is used to obtain a synergistic effect principally focussed on the thermal stability of water-blown rigid polyurethane (RPU) foams. Density, microcellular structure, thermal stability and thermal conductivity are examined for RPU foams reinforced with different contents of SFG (0, as reference material, 2, 4 and 6 wt%). The sample with 6 wt% SFG presents a slightly thermal stability improvement, although its cellular structure is deteriorated in comparison with the reference material. Furthermore, the influence of SFG particles on chemical reactions during the foaming process is studied by FTIR spectroscopy. The information obtained from the chemical reactions and from isocyanate consumption is used to optimize the formulation of the foam with 6 wt% SFG. Additionally, in order to determine the effects of functionalization on SFG, foams containing only silane-modified nanosepiolites, only graphite, or silane-modified nanosepiolites and graphite added separately are studied here as well. In conclusion, the inclusion of SFG in RPU foams allows the best performance to be achieved.

Cryogenic thermal insulating and mechanical properties of rigid polyurethane foams blown with hydrofluoroolefin: Effect of perfluoroalkane

The effect of perfluoroalkane (PFA) on the morphology, thermal conductivity, mechanical properties and thermal stability of rigid polyurethane (PU) foams was investigated under ambient and cryogenic conditions. The PU foams were blown with hydrofluorolefin. Morphological results showed that the minimum cell size (153 μm) was observed when the PFA content was 1.0 part per hundred polyols by weight (php). This was due to the lower surface tension of the mixed polyol solution when the PFA content was 1.0 php. The thermal conductivity of PU foams measured under ambient (0.0215 W/mK) and cryogenic (0.0179 W/mK at −100°C) conditions reached a minimum when the PFA content was 1.0 php. The low value of thermal conductivity was a result of the small cell size of the foams. The above results suggest that PFA acted as a nucleating agent to enhanced the thermal insulation properties of PU foams. The compressive and shear strengths of the PU foams did not appreciably change with PFA content at either −170°C or 20°C. However, it shows that the mechanical strengths at −170°C and 20°C for the PU foams meet the specification. Coefficient of thermal expansion, and thermal shock tests of the PU foams showed enough thermal stability for the LNG carrier’s operation temperature. Therefore, it is suggested that the PU foams blown by HFO with the PFA addition can be used as a thermal insulation material for a conventional LNG carrier.

Effects of Various Types of Expandable Graphite and Blackcurrant Pomace on the Properties of Viscoelastic Polyurethane Foams

We investigated the effect of the type and amount of expandable graphite (EG) and blackcurrant pomace (BCP) on the flammability, thermal stability, mechanical properties, physical, and chemical structure of viscoelastic polyurethane foams (VEF). For this purpose, the polyurethane foams containing EG, BCP, and EG with BCP were obtained. The content of EG varied in the range of 3-15 per hundred polyols (php), while the BCP content was 30 php. Based on the obtained results, it was found that the additional introduction of BCPs into EG-containing composites allows for an additive effect in improving the functional properties of viscoelastic polyurethane foams. As a result, the composite containing 30 php of BCP and 15 php of EG with the largest particle size and expanded volume shows the largest change in the studied parameters (hardness (H) = 2.65 kPa (+16.2%), limiting oxygen index (LOI) = 26% (+44.4%), and peak heat release rate (pHRR) = 15.5 kW/m2 (-87.4%)). In addition, this composite was characterized by the highest char yield (m600 = 17.9% (+44.1%)). In turn, the change in mechanical properties is related to a change in the physical and chemical structure of the foams as indicated by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) analysis.

Effect of tris(1-chloro-2-propyl)phosphate in combination with aluminum hypophosphite and melamine polyphosphate on flame retardancy and thermal decomposition of rigid polyurethane foams

RPUF with tris(1-chloro-2-propyl)phosphate (TCPP), melamine polyphosphate (MPP) and aluminum hypophosphite (AHP) alone, as well as their binary and ternary blends, were prepared via a one-step process. The effect of TCPP in combination with AHP and MPP on flame retardancy and thermal decomposition in the RPUF has been investigated. The results show that adding TCPP, MPP and AHP into RPUF simultaneously can significantly ensure the uniform cell structure, enhance the compressive strength, thermal stability and fire resistance of RPUF, decrease the thermal conductivity, the release of toxic HCN at high temperature. TGA results indicate that partial substitution of TCPP with MPP and AHP could improve the char residue. When the content of TCPP is 10 wt%, the optimal ratio of MPP and DPER was 1/2, the TCPP10/MPP3.3/AHP6.7/RPUF sample reached a V1 rating in vertical UL-94 test with a limiting oxygen index of 27.4%. The compressive strength and specific compressive strength (compressive strength/density) for TCPP10/MPP3.3/AHP6.7/RPUF sample increased about 82.6% and 44.3% compared to that of pure RPUF, respectively. The cone calorimeter test results showed that adding EG, MPP and AHP into RPUF simultaneously can significantly decrease the heat release rate (HRR), total heat release (THR) and smoke emission behavior of RPUF sample. Based on these facts, a potential flame-retardant mechanism was proposed.

Comprehensive Investigation of the Behavior of Polyurethane Foams Based on Conventional Polyol and Oligo-Ester-Ether-Diol from Waste Poly(ethylene terephthalate): Fireproof Performances, Thermal Stabilities, and Physicomechanical Properties

The chemical recycling of postconsumer poly(ethylene terephthalate) (PET) bottles to produce highly thermally stable polyurethane foam (r-PUF) with excellent flame-retardant (FR) performance could be applied on an industrial scale to create a sustainable recycling industry. The advantage of oligo-ester-ether-diol obtained from waste PET glycolysis is its application in r-PUF, generating a durable foam with excellent fire resistance at rather low loadings of phosphorus-nitrogen FRs (P-N FRs), especially in high moisture environments. Compared to polyurethane foam from commercial polyol (c-PUF), r-PUF is notably more thermally stable and efficient in terms of flame retardancy, even without adding FRs. By incorporating 15 php diammonium phosphate (DAP) as a P-N FR, r-PUF/DAP self-extinguished 5 s after the removal of the 2nd flame application with a limited oxygen index value of 24%. However, for c-PUF, a much higher DAP (30 php) loading did not exhibit any rating in the vertical burning test. The aromatic moiety in the oligo-ester-ether-diol structure strongly enhanced the compressive strength and thermal stability. The positive outcomes of this study also confirmed that the r-PUF/DAP prepared from oligo-ester-ether-diol not only satisfied the fire safety requirements of polymer applications but also contained a high percentage of postconsumer PET, which could help reduce the amount of recycled polymer materials and improve waste management.

Enhanced Fire Safety of Rigid Polyurethane Foam via Synergistic Effect of Phosphorus/Nitrogen Compounds and Expandable Graphite

In order to explore highly efficient flame-retardant rigid polyurethane foam (RPUF), phosphorus/nitrogen compounds and expandable graphite (EG) were successfully incorporated into RPUF by a free one-spot method. The combustion results showed that the fire safety of the RPUF samples was remarkably improved by the addition of phosphoric/nitrogen compounds and EG. With the incorporation of 22.4 wt.% phosphorus/nitrogen compounds and 3.2 wt.% EG, the RPUF composites achieved UL-94 V-0 rating. Besides, the total heat release and total smoke release of RPUF composites were reduced by 29.6% and 32.4% respectively, compared to those of the pure RPUF sample. PO• and PO2• together with nonflammable gaseous products were evolved from phosphoric/nitrogen compounds in the gas phase, which quenched the flammable free radicals in the matrix and diluted the concentration of combustible gaseous products generated from PRUF during combustion. The compact char residues which acted as excellent physical barriers were formed by catalysis of EG and phosphoric/nitrogen compounds in the condense phase. The fire hazard of RPUF was significantly reduced by the synergistic effect of phosphorus-nitrogen compounds and EG. This work provides a promising strategy to enhance the fire safety of RPUF.