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This study investigates the postfire mechanical properties of epoxy glass-fiber
reinforced composites (EP GFRCs) using increasing concentrations of ammonium polyphosphate (APP) and inorganic silicate (InSi) to modify the char and fire residue. A facile chocolate bar-inspired structure was introduced for fire exposure and subsequent flexural testing of the GFRCs. The resin matrix used here was a diglycidyl ether of bisphenol-A (DGEBA) resin, cured with dicyandiamide (DICY), and accelerated by Urone. The microstructures of the degraded composites after three-point bending tests, were evaluated using scanning electron microscopy (SEM) and x-ray computed tomography (XCT) imaging. A previous study showed that increasing the APP and InSi content significantly enhanced flame retardancy, via improved char formation under fire conditions. However, flexural properties and fire resistance were adversely affected after fire exposure, highlighting a trade-off effect. Fiber breakage and delamination of the composites increased upon failure with increasing APP + InSi content in the composite due to unconsolidated char. The experimental values for the postfire flexural mechanics were in good agreement with the two-layer model proposed in literature. This paper presents a preliminary basis for postfire mechanical testing of epoxy composites for use in fire-safe structures, using a combination of standardized testing norms.
This study systematically investigates the transfer of flame retardants (FRs) from epoxy resins to composites. The flame-retardant composites are formulated using bisphenol A diglycidyl ether as the resin and dicyandiamide as the hardener, reinforced with glass fibers (GFs) using various textile architectures: unidirectional (UD), bidirectional (BD), and woven rovings (WR). These composites are evaluated using bench-scale fire stability tests and cone calorimeter experiments to assess critical parameters, including the temperature at failure, time to failure, and fire behavior. Among the tested configurations, UD-GFs demonstrate superior flame retardancy, fire stability, flammability, and mechanical performance, attributed to their higher residue yield, forming a more efficient protective char layer. However, the addition of FRs is limited by their impact on the material's mechanical properties. When the FR content increases to 30 and 50 wt.% of the resin, the composites exhibit a decrease in mechanical performance, adversely affecting both time to failure and temperature at failure. While adding FRs reduces the risk of fire propagation, it does not substantially enhance fire stability or mechanical performance.
This is a corrigendum to the original article "Investigating the changing dynamics of processing, temperature-based mechanics, and flame retardancy in the transfer of ammonium polyphosphate/inorganic silicate flame retardants from epoxy resins to glass fiber composites" that was published in the "Journal of applied polymer science", vol. 141 (2024), article no. e55988.
The systematic transfer of solvent-free, additive flame retardant (FR) formulations from epoxy resins to glass fiber-reinforced epoxy composites (GFRECs) through prepregs is difficult. Additionally, obtaining data on their post-fire mechanics is often challenging. Utilizing melamine polyphosphate (MPP), ammonium polyphosphate (APP), and silane-coated ammonium polyphosphate (SiAPP) FRs with low-melting inorganic silicates (InSi) in an 8:2 proportion and 10% loading by weight in a diglycidyl ether of bisphenol A (DGEBA) resin, a systematic investigation of the processing properties, room-temperature mechanics, and temperature-based mechanics of the systems was performed. The resin was cured with a dicyandiamide hardener (DICY) and a urone accelerator. The results revealed no substantial impact of these FRs at the current loading on the resin's glass transition temperature or processability. However, the fire residues from cone calorimetry tests of the composites containing FRs were found to be only 15-20% of the thickness of the resins, implying a suppression of intumescence upon transfer. At room temperature, the decrease in the flexural modulus for the composites containing FRs was negligible. Exposure of the composites in a furnace at 400°C as a preliminary study before ignition tests was shown to cause significant flexural moduli reductions after 2.5 min of exposure and complete delamination after 3 min making further testing unviable. This study emphasizes the need for future research on recovering modes of action upon transfer of FR formulations from resins to composites. Based on the challenges outlined in this investigation, sample adaptation methods for post-fire analysis will be developed in a future study.
Although numerous investigations study the improvement of flame retardancy of epoxy resins using additives, maintaining the flame retardant (FRs) modes of action present in the resins upon transfer to composites is challenging. In this study, ammonium polyphosphate (APP) and inorganic silicate (InSi) are loaded at 10%, 30%, and 50% by weight, in a diglycidyl ether of bisphenol A (DGEBA) resin cured with dicyandiamide and transferred to bidirectional (BD) glass fiber (GF) composites. Although a 50% loading of the FRs impacts the curing kinetics of the resin system, the effect on the glass transition temperature of the resin system remains negligible compared to reactive FRs in the state of the art integrated into the resin's chemical structure. Increasing the FR content improved the heat release characteristics in both the resins and composites. However, the charring mode of action is completely suppressed in the formulation with 10% APP + InSi. A 30% concentration of the FRs restored the charring action in the composite and the GFs provide increased protective layer action upon transfer to the composites. This study highlights the importance of accounting for the changing dynamics related to processing and flame retardancy upon transferring FRs from resins to composites.
Epoxy (EP) glass fiber reinforced composites (GFRCs) are extensively used in structural applications due to their excellent thermal and mechanical properties, but their inherent flammability limits fire-safe deployment. While numerous studies examine the flame retardancy of resins or individual composite systems, comprehensive studies evaluating simultaneous improvements in flame retardancy and post-fire mechanical integrity, specifically through prepreg processing, remain challenging. This study investigates the effectiveness and transferability of phosphorus-based flame retardant (FR) systems to diglycidyl ether of bisphenol A (DGEBA) and EP novolac resin matrices and their corresponding bidirectional glass fiber composites via prepregs. The FRs are chosen based on varying modes of action: ammonium polyphosphate with inorganic silicate (APP/InSi) primarily acts in the condensed phase (CP), and aluminum diethyl phosphinate with zinc hydroxystannate (AldietPO2/ZHS) is known to demonstrate both gas-phase (GP) and CP activity. Fire residues are tailored to compensate for structural defects from fire exposure. EP novolac, with higher aromaticity and cross-linking, possesses better inherent flame resistance compared to DGEBA. The novolac composites containing AldietPO2/ZHS simultaneously showed the highest retention of flexural properties after fire exposure and the best fire safety index. The experimental values of the post-fire flexural properties in the composites calibrated damage parameters in two theoretical models.
This study examines the transfer of the flame-retardant aluminum diethyl phosphinate (AlPi) from epoxy resins to composites and the impact of AlPi on fire behavior and fire stability. Further, the effects of different particle sizes and the addition of zinc hydroxy stannate (ZHS) and inorganic silicate (InSi) are investigated. The research provides a detailed analysis of the fire behavior, fire stability, pyrolysis, flame-retardant modes of action, and flame retardancy index. Interestingly, the particle sizes did not significantly affect the flammability, fire behavior, or fire stability in the systems investigated. AlPi acts primarily in the gas phase, releasing phosphorus that yields flame inhibition. Adding glass fibers (GFs) to the epoxy resin boosts the flame retardancy in the condensed phase, reducing the heat release rate (HRR), total heat release (THR), and peak heat release rate (PHRR) by about 60%. This improvement is largely due to replacing fuel and forming a protective layer during burning. Despite the differences between thermoset and composite, the study demonstrates an effective transfer of flame-retardant properties from epoxy resins to fiber-reinforced composites, significantly enhancing the flame retardancy performance in both material systems.
The residual post-fire mechanical properties of fiber-reinforced epoxy composites are influenced by their fire residues after burning. This study uses intumescent/low-melting glass flame retardants to tailor fire residues in epoxy resin. Processibility of prepregs and their quality are analysed for transfer of the flame-retardant epoxy resins to layered glass-fiber reinforced composites. Minimal effects were found on the pre-fire flexural strengths of the composites due to low loading of the flame retardants. However, when transferred to glass-fiber reinforced composites, the fire residues diminish significantly. Further studies are required to improve theoretical and experimental estimations of the post-fire mechanics of the composites.
The transfer of flame-retardant epoxy resin formulations to composites for structural and lightweight applications in the energy and transport sector can be complex and requires systematic investigation. Here, uncoated and silane-coated ammonium polyphosphate (APP, Si-APP) intumescent compounds are transferred to prepregs using a Bisphenol A diglycidyl ether (DGEBA) resin matrix to 0/90o bidirectional glass fibers with an aerial weight of 600 g/m2. Inorganic silicates (InSi) are added to improve the fire residues of the composites after combustion and the total amount of flame retardants in the resin formulations was 10% by weight. The prepregs' fiber volume content (FVC) is corroborated using in-process measurements and thermogravimetric analysis. This was found to be approximately 60% for the composites containing APP, which is ideal for load-bearing applications with high mechanical strength. However, the Si-APP containing prepregs showed an inconsistent FVC potentially due to interactions with the commercial sizing of the glass fibers. The composites subsequently prepared are analysed for potential filtration effects of the additives via SEM after immobilization in a cured resin and platinum sputtering. The inter-laminar shear strength and the mechanical strength of the composites are compared via three-point bending tests. Additionally, the fire performance of the composites is analysed using cone calorimetry. It was found there was minimal impact of the loading of the flame retardants on the delamination of the composites and the flexural strength. However, the high density of the glass fibers possibly results in a suppression of the mode of action of the flame retardants in the composites. Therefore, the fire residues of the composites are further compared using SEM, and the flame-retardant modes of action upon transfer are investigated.
The residual post-fire mechanical properties of fiber-reinforced epoxy composites are influenced by their fire residues after burning. This study uses intumescent/low-melting glass flame retardants to tailor fire residues in epoxy resin. Processibility of prepregs and their quality are analysed for transfer of the flame-retardant epoxy resins to layered glass-fiber reinforced composites. Minimal effects were found on the pre-fire flexural strengths of the composites due to low loading of the flame retardants. However, when transferred to glass-fiber reinforced composites, the fire residues diminish significantly. Further studies are required to improve theoretical and experimental estimations of the post-fire mechanics of the composites.
