Synthesis and synergistic effect of cuprous(I) oxide nanoparticles and polyethyleneimine modified ammonium polyphosphate on enhancing the flame resistance of epoxy resin

In this study, cuprous(I) oxide nanoparticles (Cu2ONPs) and APP@PEI (polyethyleneimine-modified ammonium polyphosphate) materials were successfully synthesized and combined to create a flame-retardant system for the fabrication of epoxy (EP) resin with high flame resistance. The characteristic results revealed that the synthesized Cu2ONPs exhibited a particle shape with a size below 200 nm. The synthesized APP@PEI material possessed dimensions ranging from 10–20 micrometers, featuring a core–shell structure. The combination of Cu2ONPs and APP@PEI has proven to be an effective flame-retardant system, significantly enhancing the flame resistance of EP resin. The epoxy-based composite prepared with 2 wt% Cu2ONPs and 18 wt% APP@PEI demonstrated high flame resistance, achieving a V-0 rating (the highest rating according to the UL-94 method for evaluating the flame retardancy of materials) and a limiting oxygen index value of 36% (indicating the material’s self-extinguishing ability). This limiting oxygen index value was significantly higher than that of neat EP resin, which only reached 19%. The characteristic results of the flame-retardant epoxy-based composite demonstrated that both APP@PEI and Cu2ONPs were well dispersed in the EP resin and did not undergo structural transformation during the material preparation process. APP@PEI and Cu2ONPs enhanced the thermal degradation of EP-based composite materials at lower temperatures, exhibiting a faster degradation rate compared to EP resin. This augmentation facilitates the formation of a protective char layer on the surface of composite, thereby shielding it from direct flame exposure during combustion. Cu2ONPs played a role as oxidative catalysts, acting as Lewis acids. In addition to enhancing fire resistance, APP@PEI and Cu2ONPs have increased thermal conductivity, reduced impact strength, maintained tensile strength, and improved flexural strength of the composite material based on EP compared to neat EP resin.

Ammonium polyphosphate (APP) is a phosphorus-based flame-retardant with high nitrogen content and remarkable thermal stability [14][15][16].However, APP is hygroscopic and has poor compatibility with polymers.This limitation has restricted the application of APP in the fabrication of flame-retardant polymer materials.An effective solution to overcome the limitations of APP is to modify it with organic compounds through a microencapsulation process such as epoxy resin [17], zeolitic imidazolate framework-67 [18], triallyl cyanurate (TAC)/SiO 2 double-layered [19] which creates a core-shell structure to enhance its water resistance and compatibility with the polymer matrix.
Tan Y et al conducted research on a multifunctional organic-inorganic hybrid based on cation exchange reactions between APP and diethylenetriamine (DETA) and alicyclic piperazine (PAz), resulting in DETA-APP [20] and PAz-APP hybrid mixtures [21].These mixtures were then used as flame-retardant additives for epoxy resin.With just 7.5 wt% content of DETA-APP or PAz-APP additive in the epoxy resin, the materials achieved a UL-94 V-0 rating.When 15 wt% of DETA-APP or PAz-APP additive was incorporated into the epoxy resin, the resulting materials had respective LOI values of 30.5% and 31.5%.Polyethyleneimine (PEI) has been shown to enhance the mechanical properties and exhibit high compatibility with epoxy resin.Nguyen F N and Berg J C observed that when PEI was integrated into the epoxy network, both the fracture toughness and stiffness were improved [22].APP was also modified with epoxy (APP@PEI) and used as a flame-retardant filler for epoxy resin [23].With a usage level of 15% APP@PEI, the resulting epoxy composite material exhibited an improvement of flame resistance, achieving a UL-94 V-0 rating and an LOI value of 29.5%.The LOI value of epoxy resin using the flame-retardant filler APP@PEI is expected to be further increased to expand the application potential of epoxy for more demanding flame resistance requirements.
Cu 2 O [17,24,25] and certain metal oxides such as TiO 2 , La 2 O 3 [26], Fe 2 O 3 , CaO [27], ZnO [27,28] have the ability to synergistically cooperate with APP to enhance the flame resistance of certain types of plastics such as epoxy and polypropylene by increasing the formation of a protective carbon layer on their surface.Cheng M J and colleagues demonstrated the effective synergistic effect of Cu 2 O with APP modified by epoxy encapsulation (commercial APP-EP containing 5 wt% epoxy modifier, particle size of 25 micrometers) in improving the flame resistance of epoxy resin [24].Epoxy resin blended with 18 wt% APP-EP and 2 wt% Cu 2 O achieved the flame resistance criteria of UL-94 V-0 and a very high LOI value of 35%.On the other hand, with a loading of 20 wt% of the filler APP-EP in epoxy resin, the LOI value only reached 31%, and the sample did not meet the UL-94 criteria.These results indicate that modifying APP with epoxy has greatly improved the compatibility of APP with epoxy resin.However, it is expected that the particle size of 25 micrometers can be further reduced to enhance the role of the commercial flame-retardant filler APP-EP.The results also clearly demonstrate the important role of Cu 2 O in enhancing the flame resistance when synergistically combined with APP-EP.
In this study, APP was modified with PEI as a flame-retardant filler combined with the cuprous(I) oxide nanoparticles (Cu 2 ONPs) additive incorporated into the epoxy resin matrix.The aim of this research was to reduce the particle size of APP@PEI to below 25 micrometers (the particle size of the commercial APP-EP material mentioned earlier [24] and to further enhance the flame resistance of epoxy resin through the synergistic effect of Cu 2 ONPs with APP@PEI, surpassing the capabilities of previous studies.

Preparation of Cu 2 O nanoparticles
The Cu 2 ONPs were synthesized according to the known procedure presented in figure 1 [29].Firstly, 1.25 g of CuSO 4 •5H 2 O was dissolved in 50 ml of distilled water to form solution A. Subsequently, 6 g of PEG was dissolved in 50 ml of water to create solution B. Solution B was then slowly added to solution A with vigorous stirring, resulting in solution C. Next, ascorbic acid (0.9 g) and NaOH (0.4 g) were dissolved in 100 ml of distilled water to form solution D. Solution D was gradually added to solution C to form solution E. Finally, solution F containing NaBH 4 (0.4 g), which had been dissolved in 50 ml of H 2 O, was slowly added to solution E. Continuous stirring was maintained until a dark red solution was obtained.To leave the dark red solution overnight, a yellow solution was obtained.The yellow solution was centrifuged at a speed of 4000 rpm to collect the solid material.The solid material was then dried overnight in a vacuum oven at 60 °C to obtain the final Cu 2 ONPs material.The product yield of Cu 2 ONPs ranged from 73 to 76%.

Preparation of APP@PEI flame-retardant filler
The APP was modified with PEI according to the reported procedure [23] that can be seen in figure 2. A mixture of ethanol (210 mL) and distilled water (10 ml) was transferred to a three-necked round-bottom flask and stirred under an argon atmosphere for 15 min.Subsequently, PEI (7.0 g) was added to the flask and stirring was continued.After 30 min, 10 g of APP was added to the three-necked flask, and the temperature of the solution was raised to 90 °C.After 4 h of reaction at 90 °C under argon gas blowing and continuous stirring, the mixture was naturally cooled to room temperature.The solid was collected by vacuum filtration, washed several times with ethanol to remove the solvent, and then dried overnight under vacuum at 80 °C to obtain the final product, APP@PEI.The product yield of APP@PEI ranged from 53 to 55%.

Preparation of flame-retardant epoxy
Epoxy resin with APP@PEI flame-retardant fillers and Cu 2 ONPs additives, was fabricated using the following procedure (presented in figure 3).First, APP@PEI and Cu 2 ONPs were dried in a vacuum oven at 80 °C for 6 h.Then, a mixture of E51, APP@PEI, and Cu 2 ONPs (as shown in table 1 with the specified usage amounts) was stirred at 60 °C until uniformly dispersed.The hardener PA650 (E51:PA650 = 1:1 by weight) was added to the mixture, and stirring was continued vigorously for an additional 3 min.Finally, the resulting mixture was poured into a mold with a thickness of 3.4 mm to form the resin sample in sheet form.

Characterization methods
Morphology and structure of the samples were characterized by the following methods: x-ray diffraction patterns (XRD) were recorded on a HUT-PCM-Bruker D8 advance instrument diffractometer system equipped with Ni-filtered Cu Ka radiation (operating at 40 kV, 40 mA, wavelength k = 0.154 nm).The scanning electron microscope (SEM) images were recorded on a Jeol JSM-7500F instrument.The sample was dried, mounted on a thin plate, and coated with a thin gold layer before recording.BET method [30] was measured by ASAP2010 equipment (Micrometrics-USA).The sample was treated in vacuum of 106 mmHg, at 120 °C for 4 h and at 350 °C for 9 h.The Fourier transform infrared spectroscopy (FTIR) spectra were recorded using an IMPACT-410 (Germany) infrared spectrophotometer at room temperature in the range of 4000-400 cm −1 .Thermal   analysis (TG-dTG, DSC) was performed in the range of 0 °C-800 °C under argon (TG-dTG) and oxygen (DSC) flows and a heating rate of 10 °C min −1 on a Labsys TG, SETARAM (France).
Flame-retardant properties of the composites were evaluated by UL-94 method [31] (on the Atlas HVUL2 Horizontal Vertical Flame Chamber according to the standard vertical burning test ASTMD 3801-96) and LOI index (on a JF-3 instrument according to GB/T 10707-2008 standard) with the sample thickness of 3.4 mm.The heat transfer rate value of the samples was measured on a Linseis Transient Hot Bridge THB 500 measuring instrument.
The izod impact (notched) strength measurement was carried out on a 402V V-Shaped Notch Maker Test Resources, USA, according to ASTM D256 with specimen dimensions of 64 × 12.7 × 3.2 mm 3 .The depth under the notch of the specimen is 10.2 mm.For the result of each formulation, the average of five measurements was reported.
The flexural strength test was done on a 3-point bend fixtures machine G1095, according to ASTM D790 at a crosshead speed of 2.8 mm min −1 with specimen dimensions of 150 × 12.7 × 10 mm 3 .The test is ceased if the sample reaches 5% deflection or breaks before reaching 5%.The average of three measurements was reported.
The tensile strength test was perfomed on a GT-7010-D2ELP Gotech Testing Machine, Taiwan) according ASTM D638 (at a crosshead speed of 5 mm min −1 and temperature of 23 °C).The specimen were cut into dumbbell-shaped with a Gauge length of 50 mm.The average of three measurements was reported.

Characterization of APP@PEI material
The IR spectra of APP, PEI, and APP@PEI samples are illustrated in figures 7 and S1.The characteristic peaks of APP at wavenumbers of 3166.14 cm −1 (N-H), 1265.41 cm −1 (P=O), 1087.18cm −1 (symmetric P-O stretching), 1026.51 cm −1 (symmetric stretching of PO 2 and PO 3 ), and 890.47 cm −1 (asymmetric P-O stretching) can be observed [23,34].The IR spectrum of the APP@PEI sample still exhibits all the characteristic peaks of APP, albeit with less sharpness.Additionally, the IR spectrum of APP@PEI shows additional peaks at wavenumbers of  + group of APP and the NH 2 group of PEI [23].
The morphological characteristics of the APP particles after modification with PEI are observed in the SEM images presented in figure 8.The SEM images of the APP material (figures 8(a), (b)) show that before modification, the APP material consists of particle aggregates with a rough and irregular surface, with sizes ranging from approximately 10-20 micrometers.However, after modification with PEI, a significant change in the surface of the APP@PEI material is evident.The surface of APP@PEI particles has become smooth and  uniform (figures 8(c), (d)), with particle sizes similar to those of APP particles.The clear difference in surface morphology between the APP material before and after modification with PEI indicates that PEI has coated the outer surface of the APP particles, forming a core-shell structure of the APP@PEI material.

Flame retardancy performance
In this study, the Cu 2 ONPs additive and the APP@PEI flame-retardant filler were employed to enhance the fire resistance capability of epoxy resin.Table 1, figures 9 and 10 shows the results of the fire resistance capability of the neat epoxy resin, which exhibited very low performance (unable to determine UL-94 rating and a limiting oxygen index (LOI) value of only 19%).With the addition of 20 wt% APP@PEI (without Cu 2 ONPs), the fire resistance of the material was significantly improved.The EP/20APP@PEI sample achieved an LOI value of 31%, making it a self-extinguishing material (LOI index of 27% is the required value for a material to be considered self-extinguishing [35,36]), and it attained the highest UL-94 rating of V-0 in terms of fire resistance.The information from table 1 can provide insights into the influence of APP@PEI and Cu 2 O content on the fire resistance properties of the composite material.Specifically, when comparing the samples EP/18APP@PEI/2.0Cu 2 O and EP/15APP@PEI/2.0Cu 2 O, both containing 2 wt% Cu 2 O, the EP/18APP@PEI/2.0Cu 2 O sample with 18 wt% APP@PEI achieves a LOI value of 36%, which is higher than the EP/15APP@PEI/2.0Cu 2 O sample with 15 wt% APP@PEI, having a lower LOI value (33.5%).This result suggests that increasing the content of APP@PEI enhances the fire resistance of the composite material.
The impact of Cu 2 O content on the fire resistance of the composite material can be observed from two results.First, considering the two samples EP/19APP@PEI/1.0Cu 2 O and EP/15APP@PEI/2.0Cu 2 O, even though the EP/19APP@PEI/1.0Cu 2 O sample has a higher APP@PEI content of 19 wt%, which is 4% higher than the EP/15APP@PEI/2.0Cu 2 O sample (with 15 wt% APP@PEI), its LOI value is lower (29.7% versus 33.5%).This indicates that increasing the Cu 2 O content from 1 to 2 wt% enhances the fire resistance of the composite material.
However, when comparing the two samples EP/15APP@PEI/2.0Cu 2 O and EP/16APP@PEI/4.0Cu 2 O, increasing the Cu 2 O content from 2 wt% (EP/15APP@PEI/2.0Cu 2 O sample) to 4 wt% (EP/16APP@PEI/4.0Cu 2 O sample), despite the increase in APP@PEI content from 15 to 16 wt%, results in a reduction in the LOI value (from 33.5% to 32.4%).This suggests that a Cu 2 O content of 2 wt% is the most effective in enhancing fire resistance of the epoxy-based composite.
In addition, by varying the content of APP@PEI between 16-19 wt% and the content of Cu 2 ONPs between 1-4 wt% (with a fixed total additive content of 20 wt%), all obtained samples exhibit excellent fire resistance capabilities, achieving UL-94 V-0 flame rating and LOI values greater than 27%.The samples with Cu 2 ONPs contents of 4, 2, and 1.5 wt% all demonstrate higher LOI values than the sample using only 20 wt% APP@PEI flame-retardant filler (EP/20APP@PEI sample).Particularly, the EP/18APP@PEI/2Cu 2 O sample with 2 wt% Cu 2 ONPs achieves the highest LOI index, reaching the value of 36%.When the Cu 2 ONPs contents are below 2 wt%, the LOI values of the samples decrease (the EP/18.5APP@PEI/1.5Cu 2 O and EP/19APP@PEI/1.0Cu 2 O samples with the LOI values of 32.5 and 29.7%, respectively).These results are consistent with those obtained with a fire-retardant system comprising Cu 2 O additive combined with modified APP in epoxy resin, as reported previously [24].Therefore, the combination of 2 wt% Cu 2 ONPs and 18 wt% APP@PEI with epoxy resin forms the best fire-resistant material composition.
According to the publication by Cheng M J et al [24], using epoxy resin blended with 20 wt% APP modified with epoxy, the sample did not achieve a rating according to the UL-94 method.However, in this study, by using PEI as a modifying agent for APP, the epoxy-based composite achieved a UL-94 V-0 rating.This result demonstrates the superior performance of PEI compared to epoxy when used as a modifying agent for APP.
The report by Cheng M J et al [24] also demonstrated that when epoxy resin was blended with 18 wt% APP modified with epoxy agent and 2 wt% Cu 2 O, the resulting epoxy-based composite achieved an LOI value of 35%, which is lower than the LOI value obtained in this study (36%).This slight difference in LOI values might be attributed to the particle size of the APP@PEI material used in this study, which ranged from 10-20 micrometers and was smaller than the particle size of the APP-EP material used in Cheng M J et al's study (25 micrometers).The above analysis results indicate that APP@PEI is an effective flame-retardant filler for epoxy resin.Additionally, Cu 2 ONPs have shown excellent synergistic effects with APP modified with various agents.These findings will be further discussed and clarified in subsequent sections.
The EP/15APP@PEI/2.0Cu 2 O sample was synthesized, consisting of epoxy and APP@PEI contents similar to the best-performing sample reported in the study by Tan Y et al [23], with an additional 15 wt% of APP@PEI and 2 wt% of Cu 2 ONPs.The fire-retardant effect of Cu 2 ONPs on epoxy resin when combined with APP@PEI is clearly demonstrated by the achieved LOI value of 33.5% for the EP/15APP@PEI/2.0Cu 2 O sample, which is higher than the LOI value of the previously published EP/15APP@PEI sample (29.5%).
The improved fire resistance of EP/20APP@PEI and EP/18APP@PEI/2Cu 2 O samples is further demonstrated by the weight loss reduction curves obtained from TG-dTG analysis, as shown in figure 11, and the key results summarized in table 2. The results indicate that Cu 2 ONPs material is not degraded in the temperature range of 50 °C-800 °C under Ar gas flow conditions.The thermal degradation of epoxy resin occurs in a single stage at temperatures around 300 °C-600 °C, with a T max of 431.9 °C.The temperaturedependent degradation process of the APP@PEI material also exhibits a weight loss reduction in a single stage, occurring at temperatures range of 300 °C-600 °C.However, the residue content at 800 °C of the APP@PEI material is 65.06%, which is significantly higher than that at 650 °C of the EP sample (9.42%).This indicates that APP@PEI exhibits much higher thermal stability compared to EP.
Meanwhile, the thermal degradation process of the EP/20APP@PEI sample exhibits distinct changes and can be divided into two stages, similar to the findings reported by Ming-Jun Cheng et al [24].The first degradation stage occurs in the temperature range of 300 °C-400 °C (T max = 355.44°C) due to the release of NH 3 and H 2 O from APP@PEI, accompanied by the formation of a three-dimensional P-N-C and P-O-P network structure.The second degradation stage takes place in the temperature range of 400 °C-650 °C (T max = 448.75°C) as a result of partial degradation of the P-N-C structure formed during the first degradation stage [24], accompanied by the release of a significant amount of non-combustible gases containing nitrogen and H 2 O, catalyzed by P-OH [37,38].The the residue content at 800 °C for the EP/20APP@PEI sample is 9.81%, which is significantly higher than that for the EP sample (0.22%).These results demonstrate the role of APP@PEI as an intumescent flame-retardant, capable of forming an intumescent layer containing noncombustible gases such as nitrogen, NH 3 , and H 2 O, thereby protecting the epoxy matrix from the effects of temperature.
The thermal degradation of the EP/18APP@PEI/2Cu 2 O sample also exhibits a two-stage process.However, the first stage occurs in the temperature range of 300 °C-500 °C with a T max value of 346.08 °C, which is lower than the T max value of the first stage (300 °C-400 °C) observed for the EP/20APP@PEI sample (355.44 °C).The residue content at 400 °C for the EP/18APP@PEI/2Cu 2 O sample is 28.43%, approximately half of that for the EP/20APP@PEI sample.This result indicates that Cu 2 ONPs has an influence in accelerating the degradation rate of the material at low temperatures.On the other hand, the obtained results also demonstrate that Cu 2 ONPs impedes the subsequent degradation process of the epoxy resin.In the second stage, the degradation temperature begins at 500 °C and extends up to 800 °C, with a residue content of 13.79%,Meanwhile, the residue content of the EP/20APP@PEI sample at 800 °C is only 9.81%.The analyses conducted indicate that the addition of Cu 2 ONPs as an additive to the APP@PEI material in the epoxy matrix leads to an earlier degradation of the sample at low temperatures, resulting in the formation of a char layer combined with an intumescent layer containing non-combustible gases such as nitrogen and water vapor released by the degradation of APP@PEI.This mechanism helps protect the material from degradation at high temperatures.These results demonstrate the synergistic effect between Cu 2 ONPs and APP@PEI, demonstrating their effective flame-retardant properties in the epoxy matrix.
The DSC results presented in figure 12 once again underscore the significance of the flame retardant properties of APP@PEI and the synergistic effect when combined with Cu 2 ONPs in enhancing the fire resistance of epoxy-based composite materials.The DSC curves of the material samples exhibit peaks at different temperatures with negative enthalpy values, indicating the thermal decomposition of epoxy resin and APP@PEI as exothermic processes.Furthermore, in comparison to the Thermogravimetric Analysis (TG-dTG) curves, the DSC curves of the material samples exhibit different decomposition patterns.This distinction arises from the use of oxygen gas in DSC, as opposed to argon gas in TG-dTG.The DSC curve of pure epoxy resin shows a major exothermic peak at 486 °C and a total heat release value of approximately 727 J g −1 .Conversely, the DSC curve of EP/20APP@PEI displays a significantly higher-intensity exothermic peak at a lower temperature of 462 °C, with a total heat release value of approximately 903 J g −1 , surpassing that of pure EP sample.This result indicates that APP@PEI promotes an earlier and faster thermal decomposition process in the composite material.The rapid decomposition at a lower temperature in EP/ 20APP@PEI compared to EP sample suggests the formation of a protective char layer on the surface of EP/ 20APP@PEI, which occurs earlier than in pure EP sample.This protective layer shields the material from direct contact with flames during combustion.
In addition, the DSC curve of EP/18APP@PEI/2Cu 2 O continues to evolve, with the main exothermic peak exhibiting increasing intensity and occurring at a lower temperature of 454 °C, accompanied by a rising total heat release value of approximately 1027 J g −1 .This result highlights the role of Cu 2 ONPs as oxidative catalysts, acting as Lewis acids [24], thus accelerating and lowering the combustion temperature of the material.Consequently, an effective protective char layer forms on the surface of EP/18APP@PEI/2Cu 2 O early in the combustion process, providing excellent protection against exposure to open flames and enhancing the material's fire resistance, as demonstrated in the UL-94 and LOI test results mentioned above.
Epoxy is known for its poor thermal conductivity [39].By the incorporation of the fire-retardant system APP@PEI and Cu 2 ONPs, the thermal conductivity of the composite material based on epoxy has significantly increased from 0.1851 W/(mK) (EP sample) to 0.3492 W/(mK) (EP/20APP@PEI sample) and 0.4216 W/(mK) (EP/18APP@PEI/2.0Cu 2 O sample) (table 3).These results demonstrate that both APP@PEI and Cu 2 ONPs have the capability to enhance the thermal conductivity of epoxy resin, thereby expanding the potential applications of epoxy-based composite materials with fire resistance properties in applications requiring high thermal conductivity.

Characteristics and mechanical properties
The XRD pattern of the EP/18APP@PEI/2Cu 2 O material shown in figure 13(d) displays distinct diffraction peaks characteristic of the APP@PEI structure (2θ values of 14.8, 16.2, 23.2, 25.5, 27.5, and 39.3 °) as can be seen in the XRD pattern of the EP/20APP@PEI sample (figure 13(c)) [40], as well as peaks at 2θ values of 30°, 36.5°,42.5°, and 61.5°, which are indicative of the Cu 2 O crystalline phase [29].This confirms that the epoxy matrix and the added components retain their original structures during the fabrication process of the flame-retardant epoxy resin.The SEM image of the samples is shown in figure 14.The surface of EP sample (figures 14(a), (b)) is smooth with no pores.The APP@PEI particles have a relatively large size, making them easily distinguishable on the surface of the 18APP@PEI/2Cu 2 O/EP sample (figures 14(c), (d)).In contrast, the Cu 2 ONPs particles have a nanometer-sized scale that is difficult to observe in these SEM images.The SEM images also reveal that the 18APP@PEI/2Cu 2 O/EP sample has a smooth surface with minimal observable pore formation, indicating that the APP@PEI and Cu 2 ONPs particles are effectively dispersed within the epoxy resin matrix.This is attributed to the nanoscale size of Cu 2 ONPs and the good compatibility of APP@PEI with epoxy resin.The obtained results demonstrate that the combination of Cu 2 ONPs with APP@PEI is a favorable choice for fabricating flameretardant epoxy-based composite with excellent flame-retardant properties.The utilization of the APP@PEI flame retardant system in combination with Cu 2 ONPs has significantly enhanced the fire resistance capabilities of the epoxy-based composite materials.However, the mechanical properties of the composite materials have also undergone significant changes compared to the original epoxy resin.The results of the mechanical properties of the samples are presented in table 4 and figures S2-S10 in the supporting information.
The obtained results indicate a substantial decrease in the impact strength of the EP/18APP@PEI samples using the APP@PEI and Cu 2 ONPs flame retardant, reducing by approximately 33% compared to the original EP resin.Meanwhile, the influence of Cu 2 ONPs on the impact strength of the composite material is negligible, with the impact strength of the EP/18APP@PEI/2.0Cu 2 O sample reaching 1.36 kJ m −2 , slightly lower but not significantly different from the impact strength of the EP/20APP@PEI sample, which achieved 1.38 kJ m −2 .
However, the tensile strength results for the composite material samples show no negative impact, and even a slight increase, though not statistically significant.The tensile strength of the EP/18APP@PEI/2.0Cu 2 O sample reaches 21.71 MPa, slightly higher than the tensile strength of the original EP resin (20.12 MPa).Furthermore, when analyzing the flexural strength, the results demonstrate a significant improvement due to the use of the flame retardant system, with both flexural strength and modulus increasing.In this case, Cu 2 ONPs also plays a substantial role in improving the mechanical properties of epoxy [41], as evident when comparing the flexural strength and modulus analysis results of the two samples: EP/20APP@PEI (30.85 MPa and 1171.98MPa) and EP/18APP@PEI/2.0Cu 2 O (37.53 MPa and 1339.88MPa).
These results indicate that even at a low concentration (2 wt%), Cu 2 O has a significant beneficial effect when combined with the APP@PEI flame retardant, improving both fire resistance and mechanical properties of the epoxy-based composite material.

Conclusion
This research has yielded valuable results demonstrating the synergistic effect between APP@PEI and Cu 2 ONPs in the formation of an effective fire-retardant mixture when used in epoxy resin, resulting in epoxy-based composite material with high fire resistance.The achieved parameters include a very high LOI value of up to 36% and a UL-94 V-0 rating.Cu 2 ONPs acts as an oxidation catalyst, when combined with APP@PEI prepared with a core-shell structure, not only enhances the fire resistance of the epoxy resin-based composite material but also helps maintain and improve some material properties such as thermal conductivity (increased by nearly 2.3 times), tensile strength (insignificantly increased), and flexural strength (increased by 1.58 times), thereby increasing the applicability of the fire-resistant material based on epoxy resin.

Figure 1 .
Figure 1.The synthesis procedure of the Cu 2 ONPs sample.

Figure 2 .
Figure 2. The preparation procedure of the APP@PEI sample.

Figure 3 .
Figure 3.The preparation procedure of the flame-retardant epoxy material.

Figure 4 .
Figure 4.The x-ray diffraction pattern of the Cu 2 ONPs sample.

cubic Cu 2 O
(indexed according to the PDF file No.78e2076) with high intensity and sharpness[29,32].Additionally, the XRD pattern reveals a minor and negligible amount of metallic Cu nanocrystals, evident by scattered peaks at 2θ positions of 43.4,50.5, and 73.7°[33].These results indicate a partial reduction of Cu 2 O to metallic Cu nanocrystals, albeit with a low and insignificant content.The excessively low content of metallic Cu nanocrystals may insignificantly affect the flame resistance properties of the epoxy-based composite.SEM images of the Cu 2 ONPs crystals are shown in figure 5.The results indicate that the particles exhibit a polyhedral shape with sizes mainly below 200 nm.The synthesis and utilization of Cu 2 O in nanoparticle size as an additive combined with the flame-retardant filler APP@PEI are expected to enhance the effectiveness of epoxy-based composite fabrication with high fire resistance capabilities.

Figure 6 (
a) represents the N 2 adsorption/desorption isotherm curves of the Cu 2 ONPs sample.The synthesized Cu 2 ONPs sample exhibits a low specific surface area of 6 m 2 g −1 .Additionally, figure 6(b) shows the pore size distribution curve, indicating the concentration of pore distribution within the range of 0.8 nm.The pore volume of the Cu 2 ONPs sample is significantly low, measuring 0.03 cm 3 g −1 .These results indicate that the Cu 2 ONPs sample exists in a particulate form.

Figure 6 .
Figure 6.BET results of the Cu 2 ONPs sample.(a) The N 2 adsorption/desorption isotherm curves, (b) The pore size distribution curve.

Figure 7 .
Figure 7. FTIR spectra of the samples.

Figure 9 .
Figure 9. LOI values of the samples.

Figure 10 .
Figure 10.Images of the samples before and after the UL-94 test.

Figure 11 .
Figure 11.TG-dTG curves of the samples at the heating rate of 10 °C min −1 in Ar flow.

Table 1 .
Flame retardance properties of the samples.

Table 2 .
TG-dTG data of the samples at the heating rate of 10 °C min −1 in Ar flow.

Table 3 .
Heat transfer rate data of the samples.

Table 4 .
Mechanical properties of the samples.