The incorporation of commercial flame retardants into fiber-reinforced polymer (FRP) composites has been proposed as a potential solution to improve the latter’s poor flame resistance. However, this approach often poses a challenge, as it can adversely affect the mechanical properties of the FRP. Thus, balancing the need for improved flame resistance with the preservation of mechanical integrity remains a complex issue in FRP research. Addressing this critical concern, this study introduces a novel additive system featuring a combination of one-dimensional (1D) hollow tubular structured halloysite nanotubes (HNTs) and two-dimensional (2D) polygonal flake-shaped nano kaolinite (NKN). By employing a 1D/2D hybrid kaolinite nanoclay system, this research aims to simultaneously improve the flame retardancy and mechanical properties. This innovative approach offers several advantages. During combustion and pyrolysis processes, the 1D/2D hybrid kaolinite nanoclay system proves effective in reducing heat release and volatile leaching. Furthermore, the system facilitates the formation of reinforcing skeletons through a crosslinking mechanism during pyrolysis, resulting in the development of a compact char layer. This char layer acts as a protective barrier, enhancing the material’s resistance to heat and flames. In terms of mechanical properties, the multilayered polygonal flake-shaped 2D NKN plays a crucial role by impeding the formation of cracks that typically arise from vulnerable areas, such as adhesive phase particles. Simultaneously, the 1D HNT bridges these cracks within the matrix, ensuring the structural integrity of the composite material. In an optimal scenario, the homogeneously distributed 1D/2D hybrid kaolinite nanoclays exhibit remarkable results, with a 51.0% improvement in mode II fracture toughness (GIIC), indicating increased resistance to crack propagation. In addition, there is a 34.5% reduction in total heat release, signifying improved flame retardancy. This study represents a significant step forward in the field of composite materials. The innovative use of hybrid low-dimensional nanomaterials offers a promising avenue for the development of multifunctional composites. By carefully designing and incorporating these nanoclays, researchers can potentially create a new generation of FRP composites that excel in both flame resistance and mechanical strength.
Fiber-reinforced polymers (FRPs) are composite materials comprising a polymer matrix and fiber reinforcement. Their growing market size is attributed to their high specific modulus, strength, and toughness compared with traditional materials such as metal and ceramic [1], [2], [3], [4]. Basalt fibers, derived from natural solidified igneous rocks, exhibit exceptional mechanical properties, excellent chemical resistance, and thermostability [5]. Notably, the basalt fiber synthesis process does not require additives, making it environmentally friendly, sustainable, and cost-effective. Due to their outstanding properties and eco-friendly nature, basalt fibers are emerging as alternatives to conventional carbon and glass fibers [6], [7], [8]. The minimal waste and pollution generated during basalt fiber synthesis position them as a promising solution to the pressing issue of “carbon neutralization.” Basalt-fiber-reinforced polymers (BFRPs) have found promising applications in the marine industry, transportation structures, and civil constructions. For example, BFRP bars were successfully utilized in the Sam Thompson Bridge in Belfast in United Kingdom [9]. Epoxy, which is widely used due to its availability, processability, and good mechanical properties, is currently the predominant thermosetting polymer [10], [11]. However, epoxy’s weak thermostability limits its applications under elevated temperatures [12], [13], [14]. Moreover, most epoxy resins are flammable in the presence of air, emitting hazardous gases and thereby posing risks to humans and the environment. Numerous studies have attempted to improve epoxy’s thermostability and flame resistance through covalent modification, flame retardant incorporation, and nano-additives [15], [16], [17], [18], [19]. Chen et al. [20] fabricated a phosphorus-containing flame retardant (HBAEA-DOPO, where HBAEA is bis[2-(4-hydroxybenzylideneamino)ethyl]amine and DOPO is 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide) for epoxy resin with nano-SiO2 as the reinforcement. While the thermal stability and the temperature at the maximum weight loss rate significantly improved, the bending strength experienced a nearly 50% reduction with the nano-additives. Suihkonen et al. [21] improved the flame retardancy of epoxy-based composites by incorporating meso- and nano-sized magnesium hydroxide (MDH) particles. Despite better dispersion and interfacial bonding with the nano-sized MDH, the mechanical properties were compromised. The substantial deterioration in mechanical properties is a major limitation of commercial flame retardants. Achieving synchronized improvement of both the flame retardancy and mechanical properties of FRPs has become an urgent challenge [22], [23].
In recent decades, the field of nanotechnology has witnessed significant advancements, drawing increasing attention to low-dimensional nanomaterials [24], [25], [26], [27], [28]. Low-dimensional nanomaterials are defined as materials with at least one length dimension falling within the range of 1-100 nm [29]. Based on their shapes and structures, these materials are categorized as zero-dimensional (0D) materials, which include nanoparticles, clusters, and quantum dots; one-dimensional (1D) materials such as wires, tubes, rods, and belts; and two-dimensional (2D) materials including thin films and lattices [30], [31]. Due to their reduced size, low-dimensional nanomaterials exhibit robust mechanical properties and high flexibility [32]. Researchers have demonstrated the remarkable improvement in the mechanical properties of polymeric composites by incorporating 1D carbon nanotubes (CNTs) and 2D graphene. These advancements signify the potential of low-dimensional nanomaterials to improve the mechanical characteristics of composite materials [33], [34].
Nano kaolinite (NKN, Al2Si2O5(OH)4) and halloysite nanotubes (HNTs, Al2Si2O5(OH)4·2H2O) are two differently structured natural nanoclay materials derived from kaolin subgroups [35], [36], [37], [38]. The 2D NKN exhibits a multilayered polygonal flake shape, whereas 1D HNTs demonstrate a continuous multi-walled tubular structure [39], [40], [41]. Tang et al. [42] found that the fire retardant performance of polypropylene (PP) can be significantly improved by incorporating an intumescent flame retardant (IFR) with nano-rolled NKN and exfoliated NKN. The intumescent char strength was improved with the incorporation of NKN through its barrier and pillar-like support effects. Wang et al. [43] improved the flame retardancy of poly(butylene succinate) (PBS) by incorporating HNTs as a synergistic agent with other IFRs. The addition of HNTs increased the weight ratio of the char residue, and a tighter protective char layer was observed after burning. Moreover, as described by Wang et al. [44], the use of HNTs as a synergistic flame retardant is a promising strategy to prepare environmentally friendly intumescent flame retardants (EIFRs). Sun et al. [45] incorporated HNT and NKN together into PP containing IFR and found that the mixture of NKN/HNT improved the thermostability and flame retardancy. The researchers suggested that the NKN/HNT combination was beneficial to forming a crosslinked network and thus resulted in compact char. In addition to their outstanding synergistic flame retardancy, NKN and HNT are attractive nanomaterials for enhancing the mechanical properties of FRPs. On the basis of the high stiffness and strength of NKN and HNT, researchers have attempted to incorporate them with FRPs to improve the latter’s mechanical properties [46], [47], [48]. Our previous studies investigated the static and dynamic mechanical properties of differently structured nanoclay-incorporating BFRPs via a combination of experimental and computational approaches, revealing remarkable improvements in the properties of stiffness, strength, and damping [48], [49]. From an application perspective, nanoclay-incorporating BFRPs show promise for use in marine industries, particularly in applications such as offshore oil platforms and pipelines, as indicated by recent research findings [50], [51], [52].
In contrast to the conventional random dispersion of nano-additives within resin, recent years have seen a surge in interest in an innovative approach: the hierarchical distribution of nano-additives [53], [54], [55]. A hierarchical distribution refers to a specific dispersion of nano-additives in the interfacial region between fibers and the matrix—a technique proven to improve the interfacial and interlaminar strength of the fiber matrix [56], [57]. Improved interlaminar shear strength (ILSS) and increased interlaminar fracture toughness have been achieved via the hierarchical incorporation of CNTs into FRPs, as reported by An et al. [58], Wang et al. [59], and Liu et al. [60]. However, the impact of a hierarchical distribution on the flame retardancy of FRPs has been unexplored thus far. In this study, we devised a 1D/2D hybrid kaolinite nanoclay system to mitigate the adverse effects of conventional flame retardants on mechanical properties while simultaneously reducing flame retardant usage. This innovative additive system was engineered to possess superior flame retardant and mechanical properties.
To emphasize the synergistic flame retardancy of the 1D/2D hybrid kaolinite nanoclay system with IFRs, we introduced a novel experimental strategy to modify the distribution of the nanoclays in BFRPs. The concept of a homogeneous and hierarchical distribution of nanoclays is depicted in Fig. 1. By hierarchically distributing the 1D/2D nanoclays, we systematically investigated the roles of each component in combustion and stress transfer. Our approach involved a combination of experimental analyses and thermal degradation kinetic studies, which provided comprehensive insights into the mechanisms involved.
2. Experimental works
2.1. Materials
Pristine HNT and NKN with identical Chemical Abstracts Service (CAS) numbers (1332-58-7) were procured from Sigma-Aldrich Co., Ltd. (China). An optimized hybrid mixture consisting of 80% NKN and 20% HNT by weight, determined through preliminary orthogonal experimental studies, was employed as the synergistic flame retardant nanoclays in this research. Comprehensive characterizations of HNT and NKN can be found in Appendix A. Sodium dodecyl sulfate (SDS; Sigma-Aldrich Co., Ltd.) was used as the surfactant [61]. Unidirectional basalt fabrics, featuring a fiber diameter of 7-13 μm, were sourced from Zhengzhou Dengdian CBF Co., Ltd. (China). The IFR used was ammonium polyphosphate (APP; Sigma-Aldrich Co., Ltd.). The epoxy resin (TECHSTORM 481) and the corresponding hardener (TECHSTORM 486) were procured from TECHSTORM Co., Ltd. (China).
2.2. Hierarchical distribution of nanoclays
The hierarchical distribution of the nanoclays was achieved through electrophoretic deposition (EPD), leveraging the strong negative charges of kaolinite nanoclays [46]. Electrophoretic suspensions were prepared by thoroughly dispersing the nanoclays and an equal amount of SDS in an aqueous solution, followed by 12 h of magnetic stirring and 20 min of ultrasonication to ensure uniform dispersion [62]. The EPD process took place in a custom-made rectangular glass vessel. During EPD, a single layer of basalt fabric was positioned vertically between the anode and cathode, both made of alloy gauzes. A direct current (DC) power supply was used to apply an electric field with a strength of 800 V·m−1. Once the electric field was applied, the negatively charged nanoclays were mobilized and migrated toward the anode. Each basalt fabric underwent EPD for 5 min. After the deposition process, the basalt fabrics with hierarchically distributed nanoclays were dried at 50 °C for 24 h.
2.3. BFRP fabrication
The 12-ply BFRP laminates with a stack sequence of [0°]12 were fabricated using the hot-press technique. To ensure a uniform distribution, specific amounts of APPs and nanoclays were dispersed in the pristine epoxy resin through a 24 h sonication process. It is important to note that only APPs were dispersed in the epoxy resin for the basalt fabrics treated with EPD. Before being transferred to the hot press machine, all dry basalt fabrics were impregnated with the catalyzed resin, maintaining a weight ratio of epoxy to hardener of 10:3, using the hand lay-up method. An artificial initial crack was introduced by inserting a Teflon ply approximately at the middle of the laminate. Once all the resin-impregnated basalt fabrics were properly laminated, the hot-pressing process commenced. The forming and curing procedures were carried out at 60 °C for 0.5 h and 120 °C for 4 h, respectively. Subsequently, the fabricated BFRP laminates were cooled to room temperature and then cut into test specimens. The experimental groups, with details on various incorporation amounts and distributions, are summarized in Table 1.
2.4. Characterizations
Different experimental approaches were carried out to evaluate the flame retardant performance and mechanism of the BFRPs. A limited oxygen index (LOI) was conducted using an FTT0077 oxygen index meter (Fire Testing Technology Co., Ltd., UK) according to the American Society for Testing and Materials (ASTM) D2863 standard. The dimensions of the test specimens were 120 mm × 10 mm × 3 mm, and the orientation of the basalt fibers was aligned with the length direction. During the LOI tests, a single specimen was positioned vertically in the oxygen index meter with a flowing gas mixture of oxygen and nitrogen. The LOI was recorded as the minimum oxygen concentration required for specimen flaming combustion. The UL-94 vertical burning tests were performed using an FTT0082 instrument (Fire Testing Technology Co., Ltd.) according to the ASTM D3801 standards. A cone calorimetry test (CCT) was carried out using an FTT0007 cone calorimeter (Fire Testing Technology Co., Ltd.) according to the International Organization for Standardization (ISO) 5660 standard. Each specimen, sized 100 mm × 100 mm × 3 mm, was wrapped in aluminum foil and exposed horizontally to an external heat flux of 35 kW·m−2 sustained for 800 s. Thermogravimetric analysis coupled with infrared spectroscopy (TG-IR) tests with a heating rate of 10 °C·min−1 were performed using a combination of Nicolet iS50 (Thermo Scientific Co., Ltd., USA) and a TGA/DSC 3+ instrument (Mettler Toledo Co., Ltd., Switzerland). In addition to the TG-IR method, thermogravimetric analysis (TGA) with different heating rates of 5, 15, 20, and 30 °C·min−1 was carried out using a NETZSCH STA 449 F5 instrument (Germany) to investigate the degradation kinetics. A scanning electron microscope (SEM; S-4800, Hitachi Co., Ltd., Japan) was utilized to observe the nanoclay distributions and char morphology of the BFRPs before and after combustion, respectively.
The mechanical properties of the BFRPs were determined using a universal testing machine (KDMT-156, Kyung Do KDP Co., Ltd., Republic of Korea). Three-point bending, short beam shear (SBS), double cantilever beam (DCB), and end-notched flexure (ENF) tests were conducted according to ASTM D790, D2344, D5528, and D7905, respectively. Schematics and photographs of the EPD process and schematics and SEM photographs of different distributions are shown in Fig. 2. Additional SEM images are provided in Appendix A.
3. Results and discussion
3.1. Flame retardant properties
3.1.1. Combustion behaviors
The results of the LOI and UL-94 tests are given in Table 1. The UL-94 test provides a detailed rating system based on the burning characteristics of the material. The different ratings according to the UL-94 test are as follows:
• V0 rating: This is the highest rating. To achieve this rating, the specimen must self-extinguish within 10 s after the source of ignition is removed, and it should not have flaming drips or particles that continue to burn. Moreover, the total flaming time must be less than 50 s.
• V1 rating: Specimens with this rating must self-extinguish within 30 s after the source of ignition is removed. Like those rated V0, they should not produce flaming drips or particles, and their total flaming time should be less than 250 s.
• No rating (NR): Specimens that do not meet the criteria for V0 and V1 ratings during the UL-94 test are categorized as NR in this study.
The LOI values of the BFRPs exhibited significant increases when the BFRPs were loaded with high concentrations of both nanoclays and APP, underscoring the remarkable synergistic effects between these additives. The R-N2A6 composition was particularly noteworthy, as it displayed a 28.7% LOI increment compared with the neat BFRPs. During the UL-94 tests, no dripping phenomena were observed in any BFRP specimens. This absence of dripping can be attributed to the excellent thermal stability of the basalt fibers, which played a crucial role in stabilizing and immobilizing the viscous polymer residues post-combustion. Among the experimental groups, R-N1A6, R-N2A4, and H-N2A6 achieved a V1 rating, while R-N2A6 surpassed expectations with a V0 rating. These exceptional improvements in the LOI and UL-94 results underscore the synergistic effects arising from the incorporation of 1D/2D hybrid kaolinite nanoclays in conjunction with APP.
The CCT characterization method is a practical and powerful approach to evaluate the flame retardancy of FRPs by inducing a constant heat flux. The heat release rate (HRR), total heat release (THR), smoke production rate (SPR), and total smoke production (TSP) obtained from the CCT results were implemented to study the flame retardancy of the nanoclay-incorporating BFRPs. For example, the representative CCT results of both the homogeneous and hierarchical distributions, regarding the different nanoclay loadings and APP loadings, are illustrated in Fig. 3(a), while the corresponding photographs of the tested CCT BFRP specimens are shown in Fig. 3(b). Incorporating the nanoclays resulted in further enhancement of the flame retardancy, along with evidence of denser and more compact char layers, indicating the remarkable synergistic effect of the nanoclays and APP. Comprehensively, adding APP effectively reduced the HRR and THR of the BFRPs. This reduction effect was further enhanced with the incorporation of the nanoclays. A higher nanoclay loading resulted in more significant reduction of both the heat release and smoke production. The most effective improvement in the flame retardancy was observed for the H-N2A6 group, which exhibited reductions of 13.48%, 34.56%, 30.32%, and 42.86% in the peak heat release rate (PHRR), THR, peak smoke produce rate (PSPR), and TSP, respectively. This outstanding smoke reduction was attributed to the immobilization of free radicals and volatiles by the metal oxides contained in the kaolinite nanoclays and the encapsulation behavior of the HNT lumens. The BFRPs with homogeneously distributed nanoclays demonstrated more effective improvement of the flame retardancy compared with those with a hierarchical distribution of the same nanoclay contents. The remarkable synergistic flame retardant properties of the 1D/2D kaolinite nanoclays were noteworthy upon a comparison of the CCT results of the BFRPs with homogeneous and hierarchical distributions. For example, the more effective improvements with the homogeneous distribution were attributed to higher interaction and hyphenation opportunities with the APPs, which are vital in effectuating the synergistic flame retardancy of the nanoclays.
3.1.2. TG-IR analysis
Three-dimensional (3D) and 2D plots of the representative hyphenated infrared spectrometry results during the primary thermal degradation time are illustrated in Fig. 4. During the TG-IR tests, the produced gases and volatiles flowed into the spectrometer cell, whereby the weight change of the specimens and the hyphenated infrared spectrometry of the generated gases were recorded simultaneously. The peaks that appear at 2260 to 2405 cm−1 were assigned to carbon dioxide (CO2) [12]; the intensity of those peaks became stronger during combustion due to the production of CO2. The peak at 1508 cm−1 and the peaks at 1257, 1173, and 828 cm−1 were attributed to aromatic and ether compounds, respectively, from broken polymer chains. The new peaks that appeared at 963 and 928 cm−1 after adding the APPs indicate the P-O-P from the volatilized pyrophosphates [63]. The peaks around 2800 to 3050 cm−1 reveal the hydrocarbons that volatized from the degraded polymer chains [64]. This definition of the peaks around 2800 to 3050 cm−1 differs from that of Sun et al. [45], who suggested that these peaks were due to the vibration of the polymer chain skeleton. It is evident that the peaks that appeared at elevated temperatures were nonexistent for all test specimens at room temperature. Moreover, as the hyphenated infrared spectrometry could not examine the components of the specimens being tested in the TGA chamber, it is not very certain to determine the peaks around 2800 to 3050 cm−1 as the backbone of the polymer chains. A comparison of the infrared spectrometry curves at 280 °C showed that the volatized hydrocarbons were observed for all experimental groups except R-N2A6. The lack of hydrocarbon leaching for R-N2A6 indicates the high-temperature stability, which delayed the polymer chain degradation at elevated temperature, as well as the outstanding volatile absorption and encapsulation effects. The 1D/2D hybrid kaolinite nanoclays demonstrated significant synergistic flame retardancy with APP in the aspect of smoke reduction.
The TGA curves and the corresponding Gram-Schmidt curves are illustrated in Fig. 5. The total mass losses at 800 °C showed no significant reduction with the incorporation of nanoclays and APP. The R-N2A6 experimental group exhibited an approximately 3.0 wt% increase in mass residual compared with the neat BFRP, indicating the contribution of the incombustible nanoclays to the increased mass residual. An analysis of the derivative thermogravimetry (DTG) curves revealed a shift in the peak DTG to a lower temperature in the presence of the nanoclays and APPs, indicating a less intense pyrolysis compared with that of neat BFRP. The incorporation of the nanoclays and APPs also lowered the peak mass loss rate, signifying a milder pyrolysis process. The intensity of the total volatiles leaching was characterized using the Gram-Schmidt curves. Neat BFRP exhibited more intense volatiles released during pyrolysis compared with the BFRPs that incorporated nanoclays and APPs. Furthermore, the hierarchical distribution of nanoclays showed fewer contributions to the reduction in volatiles leaching. Hybrid kaolinite nanoclays with a homogeneous distribution revealed a significant synergistic effect in diminishing volatiles when compared with both the homogeneous and hierarchical distributed nanoclays, especially in combination with APPs.
3.1.3. SEM observations
The representative SEM photographs are exhibited in Fig. 6. It should be noted that SEM images of the fiber surfaces were captured on the bottom side of the CCT specimens, while SEM images of the char surfaces were taken from the top surfaces. In the case of R-N0A6, minimal char residue was observed on the fiber surfaces. Due to the absence of nanoclay protection, the epoxy matrix inevitably underwent pyrolysis during the CCT. However, the integrity of the basalt fibers remained well-preserved, thanks to their exceptional heat-resistant properties. R-N0A6 exhibited multi-porous char surfaces with significant porosity, which may have served as pathways for the release of generated gases and volatiles. In contrast, a char layer was formed on the fiber surfaces of H-N2A6. The hierarchically distributed nanoclays played a crucial role in anchoring the char residue on the fiber surfaces, creating a protective char layer. However, this char layer had minimal impact on delaying matrix pyrolysis and reducing volatiles. The SEM image of the char surface of H-N2A6 reveals characteristics similar to those of R-N0A6. The hierarchically distributed nanoclays appeared to have a limited synergistic effect on improving the flame retardance of the BFRPs compared with R-N0A6 and H-N2A6, aligning with the intended design of the 1D/2D hybrid nanoclay system. In contrast, the nanoclays on the fiber surfaces of R-N2A6 formed networks rather than embedding in chars. These expanded nanoclay networks significantly increased the contact area with chars, making notable contributions to the flame retardance through volatile absorption and char reinforcement. The compact char surfaces with high integrity in R-N2A6 validated the outstanding synergistic flame retardancy of the 1D/2D hybrid nanoclay system, which acted as reinforcements and char layer skeletons. The observed char formation behavior and the respective contributions of the components aligned with the initial design intention.
3.1.4. Thermal degradation kinetics
To elaborate on the synergistic flame retardancy effect of the hybrid kaolinite nanoclays, a thermal degradation kinetic analysis approach was implemented based on TGA tests with different heating rates, as shown in Fig. 7. The degradation behaviors were found to shift to higher temperatures with higher heating rates. The higher heating rates provided less energy for the BFRPs to activate the degradation, resulting in delayed decomposition. To elaborate the thermal degradation kinetics, the Flynn-Wall-Ozawa (FWO) method, which was proposed based on the Arrhenius equation and Doyle approximation, was implemented based on the non-isothermal TGA experiments [65], [66], [67], [68]. The conversion α, which is defined as the mass fraction of decomposed solid, is calculated according to Eq. (1):
$ \alpha=\frac{m_{0}-m}{m_{0}-m_{\mathrm{f}}}$
where m0 is the initial mass, m is the mass at a specific temperature, and mf is the final mass after decomposition. Then, the governing equation associated with the heating rate β and activation energy Ea can be explicitly expressed as Eq. (2):
where R is the universal gas constant, A is the pre-exponential factor, g(α) is the differential mechanism function, and T is the temperature. The activation energy Ea can then be calculated based on the slope derived from the linear fitting of lnβ versus T−1. By determining a series of conversions, the corresponding activation energy under different decomposition conditions can be obtained. The linear fittings and activation energies, with a conversion ranging from 0.2 to 0.7, are illustrated in Fig. 8. The activation energy of all representative experimental groups demonstrated an increasing tendency. The shielding effect from the well-preserved basalt fibers and the formed char layers was beneficial in protecting the vulnerable interior matrix, resulting in a comprehensive increase in the activation energy. It is noteworthy that the activation energy at different degrees of decomposition exhibited several fluctuations, indicating exposure of the interior matrix due to broken char layers. Nevertheless, R-N2A6 exhibited a relatively high and stably increased activation energy during the whole degradation stage, attributed to the integrated and compact char layer reinforced by the homogeneously distributed nanoclays.
3.1.5. Flame retardancy mechanisms
The synergistic flame retardancy mechanisms of the 1D/2D hybrid kaolinite nanoclays were hypothesized based on a combination of the material characteristics, experiments, and theoretical analysis. During the combustion and pyrolysis process, the added APPs start to decompose at elevated temperatures (> 256 °C), forming polyphosphoric acid and ammonia (NH3). The non-flammable ammonia gases moderate the combustion and intumesce the char layers. The polyphosphoric acid crosslinks with the nanoclays, which contain abundant hydroxyl groups, through dehydration reactions. These crosslinked nanoclays form networks during the char intumescence, reinforcing the char layer like a skeleton. Structurally, the 2D NKN obstructs heat transfer, while the 1D HNT reduces the robust van der Waals forces between the NKNs, bridging and reinforcing the char layer. The homogeneous distribution of the nanoclays facilitates sufficient interaction between the 1D HNT and 2D NKN, maximizing the synergistic effect. The homogeneous nanoclay distribution also improves the volatile absorption and encapsulation. The integrated nanoclay network improves the flame retardancy more effectively than the individual nanoclays. The crosslinking mechanism between the nanoclays and APPs is illustrated in Fig. 9(a). Epoxy matrix dehydration and carbonization form char layers, insulating the polymer from oxygen contact and inhibiting solid-phase combustion. Water molecules—byproducts of dehydration—improve the flame retardancy. Fig. 9(b) depicts the char-reinforcing mechanism with different nanoclay distributions. Dense networks form among the intumescent char layers in the homogeneous distribution, enhancing the synergistic flame retardancy. Sparse nanoclay networks form on the fiber surfaces in the hierarchical distribution, offering minimal interaction chances with the APPs. At elevated temperatures, the nanoclays may sinter and integrate due to the intense combustion, further reinforcing the char layers. These flame retardant mechanisms underscore the superiority of the 1D/2D hybrid nanoclay system design, which is clearly demonstrated through the controlled nanoclay distribution.
3.2. Mechanical properties
In addition to the flame retardancy, the mechanical properties of the composite materials are of primary concern. The mechanical properties, crack impediment mechanism, and comparisons of the changes in mechanical properties and flame retardancy between the current work and recently published studies are illustrated in Fig. 10 [20], [69], [70], [71], [72], [73], [74], [75], [76], [77], [78], [79], [80], [81]. The bending strength was significantly reduced with the addition of APP. Incorporating the 1D/2D hybrid nanoclays relieved the adverse effect of the APP to a certain extent for both the homogeneous and hierarchical distributions. The 2D NKN with a fake polygonal shape was preferable for impeding the cracks initiating from the APPs. At the same time, the 1D HNT with its hollow tubular structure was advantageous for spanning the cracks and bridging the epoxy matrix. Synergistic effects on the mechanical properties were achieved by an optimal hybrid of the 1D HNT and 2D NKN, resulting in the most remarkable improvement for the homogeneously distributed nanoclays, with increases in the ILSS and mode II fracture toughness (GIIC) of approximately 15% and 100%, respectively.
Despite the improvement caused by the incorporation of the nanoclays, the bending strength of the BFRPs still deteriorated with the addition of APP because the contribution of the nanoclays was limited to improving the fiber-dominated bending strength. The homogeneously distributed nanoclays exhibited a more obvious improvement in the bending strength at lower APP loadings, while the amelioration caused by the hierarchical distributed nanoclays had less impact with higher APP loadings. The homogeneous distributions inevitably led to agglomeration problems at high APP and nanoclay loadings. The agglomerated inclusions formed mesoscale clusters and may have become stress concentration sources during high deformation cases, which resulted in an unstable bending strength for the homogeneous distribution. The hierarchical distributed nanoclays had fewer chances to agglomerate with the homogeneously distributed APP in the resin, resulting in less sensitivity to different APP loading amounts.
The ILSS and GIIC, which are respectively assessed through SBS and ENF tests, are crucial parameters for evaluating the interlaminar properties of a composite material. Adding the APP impeded the bonding between the BFRP layers and thus significantly decreased the ILSS of the BFRPs. The homogeneously distributed nanoclays helped recover the ILSS as a consequence of the shielding and bridging of crack propagations by NKN and HNT. The hierarchically distributed nanoclays made fewer contributions toward the recovery of the ILSS. Nanoclays deposited by the EPD process were distributed in the interfacial regions between the fibers and the adjacent matrix, which is favorable for enhancing the fiber-matrix bonding. However, the nanoclays dispersed in the interfacial regions had a negligible effect on impeding the crack in the interlaminar region that was initiated by the APP. The GIIC results showed tendencies similar to those of the ILSS results, but the GIIC was more sensitive with the inclusion loadings. The crack-resistance curves (or R-curves) of the mode I fracture toughness (GIC) are illustrated in Fig. 10(b). The GIC values increased at the first 20 mm crack propagation length due to the fiber-bridging and interlocking behavior of the filaments [82]. Adding APP resulted in a deterioration of the GIC, and the homogeneously distributed nanoclays demonstrated an improvement in comparison with neat BFRP. The effects of the hierarchically distributed nanoclays on the GIC showed the high sensitivity of the nanoclay loadings. A 1.0 wt% hierarchical incorporation of nanoclays improved the GIC due to improvement of the fiber-matrix interfaces. In comparison, a 2.0 wt% hierarchical incorporation of nanoclays resulted in deterioration of the GIC due to agglomeration, which coincides with the results in our previous studies [48], [55].
It seems counterintuitive that our previous studies and other relative research revealed that the hierarchical distribution of nano additives is beneficial for the interlaminar properties [46], [54], [55], [56], [60], [82], [83]. Nevertheless, the weak fiber-matrix interfaces are considered to be the most vulnerable parts during delamination failure; thus, the use of hierarchical distributions is a promising method to delay delamination by increasing the fiber-matrix bonding. However, the most vulnerable parts in this study became the APP-matrix parts; therefore, the hierarchical distributions had a trifling effect on ameliorating the interlaminar properties. Combining the dynamic mechanical properties (shown in Appendix A), incorporating an appropriate amount of 1D/2D hybrid kaolinite nanoclays, and achieving synergistic effects via sufficient interaction with the APP ameliorated the mechanical properties that had deteriorated due to the APP addition, which is an encouraging finding for composite materials.
4. Conclusions
In this paper, the synergistic effects of 1D/2D hybrid kaolinite nanoclays were clearly demonstrated in load-transferring and flame-retardancy scenarios via a novel hierarchical distribution. In terms of flame retardancy, the 1D/2D hybrid kaolinite nanoclays effectively reduced heat release and volatile leaching. Hierarchical distribution was compared with homogeneous distribution, confirming the improved flame retardant performance, including a reduction in the HRR, smoke production, and activation energy. The 2D NKN forms a barrier with the intumescent char layer, reducing volatiles through 1D HNT lumen encapsulation. During pyrolysis, the 1D/2D nanoclays synergistically crosslink with the APPs through dehydration reactions. Sintering of the 1D/2D nanoclays can occur during intense combustion, amalgamating the network and reinforcing the char layer, resulting in a more compact char layer and improved flame retardancy.
Regarding load transferal, the 2D polygonal-shaped NKN impedes crack propagation through shielding, while the 1D hollow tubular HNT is beneficial for stress redistribution and matrix bridging. Both the HNT and NKN play critical roles in improving the mechanical properties of BFRPs. Despite the potential advantages of a hierarchical distribution for interfacial bonding, it has a negligible impact on inhibiting cracks initiated by APPs. Therefore, a homogeneous distribution of the 1D/2D nanoclays offers a more effective exploitation of the synergistic effects, mitigating the deterioration in mechanical properties due to APP addition.
The synergistic effects of 1D/2D hybrid kaolinite nanoclays hold promise for FRPs by enabling a simultaneous reduction in conventional flame retardant usage and improvements in both mechanical properties and flame retardancy. Hierarchical distribution serves as an effective comparison scheme, with potential applications in combustible fiber-reinforced composites, expanding the scope of plant fiber composites in the future.
Acknowledgments
This research was supported by the National Natural Science Foundation of China (11872279, 12172258, and 11625210).
Compliance with ethics guidelines
Zixuan Chen, Tianyu Yu, Zetian Yang, Zhibiao Wei, Yan Li, Weidong Yang, and Tao Yu declare that they have no conflict of interest or financial conflicts to disclose.
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