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Article

Design of a Novel Chitosan Derivatives and DOPO Flame Retardant and Its Application in Epoxy Resin

by
Yicheng Yang
,
Yue Lu
,
Wang Zhan
and
Qinghong Kong
*
School of the Environment and Safety Engineering, Jiangsu University, Zhenjiang 212013, China
*
Author to whom correspondence should be addressed.
Macromol 2025, 5(1), 9; https://doi.org/10.3390/macromol5010009
Submission received: 10 December 2024 / Revised: 26 January 2025 / Accepted: 6 February 2025 / Published: 20 February 2025
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Abstract

:
To expand the utilization of bio-based materials as flame retardants in epoxy resin (EP), a green Schiff base structural material (CSV) was synthesized via a one-pot approach employing chitosan and vanillin as the raw materials. Then, the CSV combined with 9,10-dihydro-9-oxa-10-phospha-phenanthrene-10-oxide (DOPO) (the mass ratio between CSV and DOPO was 1:2, written as CSV-DOPO) improved the flame retardancy of the EP. When the amount of CSV−DOPO in the EP was only 3 wt%, the thermogravimetric analysis (TGA) results indicated that the residue of the EP composites was 50.6% higher than that of the EP. The combustion class of the EP/3 wt% CSV−DOPO composites achieved a UL-94 V0 rating and the limit oxygen index (LOI) reached 34.0%. The cone calorimeter test (CCT) showed that the peak heat release rate (PHHR), total heat release (THR), total smoke release (TSP), and peak carbon dioxide production (PCO2P) of the EP/3 wt% CSV−DOPO composites decreased by 32.3%, 22.0%, 4.6%, and 51.0%, respectively, compared to the EP. The flame-retardancy mechanism was studied by scanning electron microscopy (SEM) and Raman spectra. The quenching effect of phosphorus-containing radicals, the dilution effect of noncombustible gases, and the impeding effect of the carbon layer in the condensed phase contributed collectively to the excellent flame retardancy of the EP/CSV−DOPO composites. Considering the facile preparation method and small addition amount of the flame retardant, the present work provides a convenient solution for the preparation of modified EP with good flame retardancy and heat stability, which is expected to be widely used in industries.

1. Introduction

Epoxy resin (EP), as a polymer possessing excellent electrical insulation properties, strong adhesion, remarkable corrosion resistance, and a high mechanical strength, finds extensive applications in composites, the coatings sector, the electronics industry for adhesives, among other fields [1,2,3]. Given that EP is a flammable substance, which restricts its utilization in certain specific areas, enhancing its fire safety characteristics is of utmost importance [4]. In response to the increasing demands of green chemistry and the principles of sustainable development, the utilization of renewable and readily degradable bio-based materials to achieve a synergistic flame-retardant effect on EP has emerged as a prominent focus in contemporary research [5,6,7]. Therefore, these bio-based materials are often used in combination with some other flame retardants.
In an attempt to address this issue, numerous researchers have incorporated 9,10-dihydro-9-oxa-10-phospha-phenanthrene-10-oxide (DOPO) in conjunction with bio-based materials into EP. DOPO is regarded as a promising phosphorus-based flame retardant on account of its non-toxic nature and high phosphorus content [8,9]. Its highly reactive P-H bonds enable the introduction of other flame-retardant elements, such as nitrogen, sulfur [10], silicon [11], boron [12], etc., into its molecular structure. These flame retardants derived from DOPO can demonstrate superior flame retardancy, for instance, a bio-based reactive flame retardant named VFD for the flame retardation of EP using vanillin (VN), furfurylamine (FA), and DOPO. When the content of VFD was 7.5 wt%, the LOI value of the EP/VFD-7.5 composite increased from that of EP by 7.5 wt%, rising from 25.8% of EP to 34.5%, and achieving a UL-94 V0 rating [13]. Meanwhile, the peak heat release rate (PHRR) decreased by 28%.
Schiff base analogs are capable of forming stable nitrogen-containing six-membered rings through the self-crosslinking reaction of the C=N functional group within their molecular structures under elevated temperature conditions [14]. This characteristic endows polymers with outstanding charring properties. Moreover, Schiff base analogs containing bio-based materials are more congruent with the environmental protection requirements of the present day. Certain bio-based materials, including vanillin, tartaric acid, chitosan, and furan, have garnered extensive attention in the preparation of EP flame retardants due to their favorable carbonization capabilities [15,16,17]. The flame retardant DPPVA was synthesized via the Schiff base reaction using vanillin, 3, 5-diamino-1, 2, 4-triazole, and diphenylphosphinic chloride. Compared with that of EP, the residue of the EP/DPPVA composites was increased to 14.9 wt% from 9.6 wt% [14]. The good flame retardancy of the material is attributed to the good crosslinking carbonization of the Schiff base structure. Given the relatively high charring content and abundant hydroxyl groups, chitosan (CS) is regarded as an effective carbonizing agent for composites [18]. A chitosan derivative CSA was synergized with DOPO to improve the flame retardance of EP. When the amount of CSA was 8 wt%, the EP composites passed the UL-94 V0 rating and the LOI value increased to 36.4%. The smoke emission value and the peak exothermic rate of the EP/CSA composites were decreased by 36.0% and 61.9%, respectively [15]. A flame-retardant CCD prepared through the combination of cinnamaldehyde, chitosan, and DOPO was added to 10 wt% CCD, and the LOI value of the EP composites was 31.6% and achieved a UL-94 V0 rating. In comparison to the EP, the total heat release rate of the EP/CCD composites was reduced by 38.8% and the total smoke emission rate was decreased by 72.0% [16]. The results indicated that it is feasible to combine DOPO with chitosan derivatives to flame-retardant EP.
Based on the charring properties of the chitosan derivative and the flame retardancy of DOPO, in this work, CSV was synthesized by incorporating vanillin into chitosan and then utilized to synergistically modify EP in combination with DOPO. To verify the successful synthesis of the CSV, the structural characteristics of the CSV compounds were identified through Fourier Transform Infrared (FT-IR) spectroscopy. To explore the impact of CSV in synergy with DOPO on the flame retardancy of EP, thermogravimetric analysis, the vertical burning test, the LOI, and cone calorimeter test were employed.

2. Materials and Methods

2.1. Materials

Epoxy resin (NPEL-128) with an epoxy equivalent weight (EEW) of approximately 188 g/equivalent was purchased from Kunshan Nanya Electronic Materials Co., Ltd. (Suzhou, China). Vanillin (AR ≥ 99.5%), anhydrous ethanol (AR ≥ 99.7%), deionized water (H2O), 4, 4-diaminodiphenylmethane (DDM, 97%), and acetone (AR ≥ 99.5%) were provided by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Chitosan (DAC > 95%) was provided by Zhenjiang Dewei Chemicals Co., Ltd. (Zhenjiang, China). DOPO was provided by Beijing Coupling Technology Co., Ltd. (Beijing, China).

2.2. Synthesis of CSV

CSV was synthesized as illustrated in Figure 1. Initially, at ambient temperature, 300 mL of anhydrous ethanol was introduced into a 500 mL round-bottom flask equipped with a magnetic stirrer and a condensing reflux tube. Subsequently, chitosan (0.06 mol, 9.672 g) was added to the flask and stirred until completely dissolved in the anhydrous ethanol. The mixture was then heated to 85 °C, followed by the addition of vanillin (0.06 mol, 9.129 g). The reaction was maintained at 85 °C under stirring for 8 h. Upon completion of the reaction, the system was gradually cooled at room temperature. The resulting product was filtered, precipitated, centrifuged, and washed three times with anhydrous ethanol. Finally, the product was dried in a vacuum drying oven at 70 °C for 24 h, and subsequently ground into a powder for further use.

2.3. Preparation of EP Composites

The preparation of the EP/CSV−DOPO composites was as follows: the EP was heated at 100 °C for approximately 60 min. Subsequently, a mixture of CSV and DOPO (in a ratio of 1:2, collectively referred to as CSV−DOPO) was added. After vigorous stirring for 60 min, DDM was introduced into the system. Finally, the mixture was poured into a clean mold and cured sequentially in a vacuum oven at 100 °C for 2 h, 130 °C for 2 h, and 150 °C for 2 h. The sample was then gradually cooled to room temperature. The formulation of the EP composites is detailed in Table 1.

2.4. Instrumental Characterization

Fourier Transform Infrared (FTIR) spectra were measured by a 6700-type spectrometer (Nicolet Instruments Co., Ltd., Madison, WI, USA), with a spectral frequency range of 400–4000 cm−1. The resolution was 4 cm−1 and the scanning rate was once per second. The vertical combustion test was performed by a CZF-type Vertical Combustion Meter (Jiangning Analytical Instruments Co., Ltd., Nanjing, China), with a composite size of 130 mm × 12.7 mm × 3.2 mm, and the test standard was by ASTM D 3801-23 [17]. The LOI was determined by a JF-3 oxygen index meter produced by Jiangning Analytical Instruments Co., Ltd. with a composite size of 130 mm × 6.5 mm × 3.2 mm, and the test standard was by ASTM D 2863-23 [17]. Thermogravimetric analysis was performed using the Setline STA thermal analyzer (Setharam Instrument Company, St Etienne, France) under a nitrogen environment, and heating from 40 °C to 700 °C, with a heating rate of 10 °C min−1; the test composite was about 5 mg. The CCT was carried out with a cone calorimeter from Stanton-Redcroft (London, UK). The size of the composite was 100 mm × 100 mm × 3 mm and the irradiated heat flux was 50 kW/m2. The composites were wrapped in aluminum foil and the data were obtained by continuous heating according to the method of ISO 5660 [19]. Under the condition of a vacuum voltage of 20 kV, a scanning electron microscope (SEM, EVO MA15) was used to observe the morphology of the char layers. The Raman spectra of the char residue were acquired with a laser confocal Raman spectrometer, the DXR RAMAN MICROSCOPE (Thermo Fisher, Waltham, MA, USA), using a helium–neon laser line at 532 nm under ambient temperature conditions.

3. Results

3.1. Structural Characterization of CSV

Figure 2 shows the FTIR spectra of the vanillin, chitosan, and CSV. Vanillin has a distinct characteristic peak near 1672 cm−1. Chitosan has a broader band in the range of 3400–3800 cm−1 due to the stretching vibrations of the O-H and N-H bonds, while the peak located at 2907 cm−1 is attributed to the C-H bond. Meanwhile, there are two distinct characteristic peaks at 1157 cm−1 and 1602 cm−1, which are attributed to the stretching vibration of the sugar structure and amino group, respectively. For the CSV, a new stretching vibration absorption peak was detected at 1645 cm−1, which was attributed to C=N, indicating the presence of a Schiff base in CSV [18]. The CSV did not show an aldehyde characteristic peak at about 1680 cm−1, indicating the absence of unreacted vanillin in the CSV.

3.2. Thermal Performance Analysis of the EP Composites

The thermal stability of the EP/CSV−DOPO composites was evaluated via TGA in nitrogen. Figure 3a presents the TGA curves of the EP composites and their corresponding samples. The specific relevant data are elaborated in Table 2. The results demonstrate that the temperatures at a 5% weight loss (T5%) and the maximum decomposition temperature (Tmax) of the EP are 370.1 °C and 390.5 °C, respectively. The T5% and Tmax of the EP/CSV−DOPO composites decreased as the doping amount of CSV−DOPO increased. After adding 3 wt% and 4 wt% of CSV−DOPO, the T5% of the EP/CSV−DOPO composites dropped to 351.7 °C and 351.1 °C, respectively. This is attributable to the fact that the stability of the O=P–O bond in the DOPO structure is inferior to that of the C-C bond in the EP [20]. When 3 wt% CSV−DOPO was added, the T5% of the EP composite reached 351.7 °C, which is higher than the T5% of the EP/3 wt% DOPO composites. This also implies that the addition of CSV does not undermine the thermal stability of the composites [21]. At 700 °C, the residue of the EP/3 wt% CSV−DOPO composite was 26.5%. The residue of the comparison sample, the EP/3 wt% DOPO composites, was 20.8%, whereas that of the EP was merely 17.0%. This is due to the fact that, when the amino groups in CSV that do not form the Schiff base structure react with the epoxy groups in the EP, they can create new linkages, which promotes the crosslinking of EP. Generally speaking, a higher crosslink density typically corresponds to a greater amount of char formation [22]. At elevated temperatures, the crosslinked structure is more effective in hindering the movement and decomposition of molecular chains, thereby enhancing the stability of char layer formation. This char layer serves to isolate the diffusion of combustible gases and volatile components into and out of the layer, thus exhibiting improved condensed-phase flame retardancy [23].

3.3. Combustion Performance of EP Composites

The results of the LOI and the vertical combustion test are summarized in Table 3. The LOI value of the EP was 23.5%. The LOI value of the EP/2 wt% CSV−DOPO composites was 29.0%, with a 23.4% increase compared to that of the EP. Moreover, the UL-94 rating of the EP/2 wt% CSV−DOPO composites reached V1. As the content of CSV−DOPO increased to 3 wt% and 4 wt%, the UL-94 rating of the EP composites reached V0, and the LOI values increased to 30.5% and 31.1%. In contrast, the comparison composite of the EP/3 wt% DOPO composites only achieved a V1 classification, with an LOI value of 30.2%. This indicates that CSV has a beneficial impact on enhancing the flame retardancy of EP composites.
To visualize the combustion process, Figure 4 presents a comparison of the vertical combustion processes of the EP and EP/3 wt% CSV−DOPO composites. As shown in Figure 4a, during the process of combustion, the EP generates a substantial amount of black smoke, and the entire combustion process lasts for a relatively long duration, consequently resulting in a greater release of smoke. In Figure 4b, the EP/3 wt% CSV−DOPO composite extinguishes itself completely within approximately 10 s. During the combustion process, only a small quantity of white mist appears, which is mainly attributed to the evaporation of water vapor produced during combustion [24]. Moreover, the EP/3 wt% CSV−DOPO composites are extinguished within a significantly shorter period, which effectively curtails the production of smoke. In contrast, as the flame burns upward, the EP composites are ultimately completely consumed by the flame. The primary amino and hydroxyl groups of chitosan in CSV reacted with EP. However, an excessive number of chitosan units may reduce the dispersion of flame retardants within the EP, consequently decreasing the crosslinking density of the cured EP. Therefore, an appropriate amount of CSV addition positively enhances the crosslinking density of EP and promotes its flame retardant efficacy.
To investigate the combustion behavior of the EP and EP composites, CCTs were conducted on the EP. The combustion parameters determined through the CCT encompass the heat release rate (HRR), total heat release (THR), smoke production rate (SPR), and total smoke volume (TSP) are illustrated in the Figure 5. The HRR curves are shown in Figure 5a. The PHRR value of the EP is 871.2 kW/m2. In comparison to the EP, the PHRR value of the EP composites gradually decreases with the incorporation of CSV−DOPO. When compared with the EP, the PHRR value of the EP/3 wt% DOPO composites dropped to 604.4 kW/m2. Meanwhile, the PHRR value of the EP/3 wt% CSV−DOPO composites decreased to 589.8 kW/m2, which was lower than that of the EP/3 wt% DOPO composites, suggesting that there exists a synergistic flame-retardant effect between CSV and DOPO. In Figure 5b, the THR value of the EP at 600 s was 154.7 MJ/m2. When 3 wt% CSV−DOPO was added, the THR value of the EP/CSV−DOPO composites was 123.7 MJ/m2, representing a 22.0% reduction compared to that of the EP [25]. Consequently, the incorporation of CSV−DOPO demonstrates a beneficial effect in suppressing the heat release of the composites.
The smoke production rate (SPR) curves are shown in Figure 5c. The results indicated that the peak of the SPR values of the EP composites gradually decreased with the increase in the content of CSV−DOPO. This suggests that CSV−DOPO has a good smoke-suppression effect during the combustion process of the EP composites. As illustrated in Figure 5d, the TSP values of the EP composites gradually decreased with the increase in the content of CSV−DOPO. The TSP of the EP at 600 s was 34.1 m2. The TSP of the EP/3 wt% CSV−DOPO composite was 32.6 m2, which was 4.6% lower than that of the EP. The EP/3 wt% DOPO composites can generate more phosphorus-containing radicals to quench the reactive radicals in the gas phase, which can better suppress smoke generation [26]. The EP composites exhibited better smoke suppression when CSV−DOPO was added. Figure 6a presents the CO2P curves of the EP and the comparison with the EP composites. From the figures, it is evident that the carbon dioxide release of the EP/3 wt% CSV−DOPO composites had a significant decrease compared to the EP, and the PCO2P of the EP/3 wt% CSV−DOPO decreased by 51.0% compared to the EP. Moreover, the EP/3 wt% CSV−DOPO composites had a significant carbon dioxide-retardation effect compared to the EP/3 wt% DOPO composites. Hence, the addition of CSV has a positive impact on the reduction in carbon dioxide gas production. The mass loss curves of the EP composites are shown in Figure 6b. The residue of the EP at 600 s was 12.5%, while the residue of the EP composites reached 18.1% with 3 wt% CSV−DOPO. The dense carbon layer formed after combustion wrapped around the surface of the EP composites, effectively isolating the polymer matrix from free-radical transfer in the gas [27].

3.4. Analysis of Char Residue

The residues of the EP composites after the CCT are illustrated in Figure 7. As shown in Figure 7a, there are pronounced cracks on the surface of the char layer of the EP. This phenomenon can be attributed to the complete degradation of the EP as it reacts with oxygen, resulting in a minimal amount of residual condensed-phase products. Consequently, the weak char layer exhibits inadequate barrier properties against oxygen infiltration [28]. The char layer on the outer surface of the EP/3 wt% CSV−DOPO composites was more dense and continuous, as shown in Figure 7c. As a contrast to the composites, the char layer surface of the EP/3 wt% DOPO composites was observed to be thinner and exhibited less char layer buildup, as illustrated in Figure 7b. The SEM images of the carbon layers of both the EP and the EP composites are presented in Figure 8. It is evident from Figure 8(a1) that the carbon layer of the EP displays numerous cracks on its surface, resulting in a morphology that is thin and friable. This observation suggests that the carbon layer possesses poor gas adiabatic properties. Conversely, as shown in Figure 8(c1), the outer surface of the carbon layer for the EP/3 wt% CSV−DOPO composites appears denser and more continuous [29]. The outer and inner surfaces of the EP/3 wt% DOPO composites, as shown in Figure 8(b1) and Figure 8(b2), exhibit less uniformity and density compared to those of the EP/CSV−DOPO composites. It is concluded that the formation of a dense, continuous coal cinder is primarily attributed to the presence of phosphorus-containing compounds derived from DOPO. Additionally, the nitrogen fragments in CSV enhance and intensify this effect.
In order to obtain more information about the residues for the EP and EP/3 wt% CSV−DOPO composites after CCT, the degree of graphitization was investigated using Raman spectroscopy. Usually, the degree of graphitization is mainly indicated by the R-value (R = ID/IG) [30]. Generally, the characteristic peak appearing at 1360 cm−1 is the D band, while the G band has a characteristic peak at 1580 cm−1 [31]. The D band mainly corresponds to the vibration of the flat-end carbon of disordered graphite or glassy carbon (disordered carbon structure), and the G band corresponds to the E2g mode of hexagonal graphite and the vibrations of the sp2-bonded carbon in graphite layers (graphite structure) [32]. As the ID/IG decreases, the graphitization increases, resulting in better flame retardancy. Figure 9 shows the Raman spectra of the residual char from the EP, EP/3 wt% DOPO, and EP/3 wt% CSV−DOPO composites. The R-value of the EP/3 wt% CSV−DOPO (2.52) composite is smaller than the R-value of the EP (3.01), which indicates that the degree of graphitization in the combustion residue of the EP/3 wt% CSV−DOPO composite is significantly increased. In addition, coke with a higher degree of graphitization exhibits a dense layer, which effectively mitigates the heat transfer from the combustion flame and mass transfer from the fuel matrix. The Raman spectrum analysis of the char residues is consistent with the findings obtained from the SEM.

3.5. Flame-Retardance Mechanism

The flame-retardant mechanism is shown in Figure 10. Combined with the UL-94, LOI, CCT, scanning electron microscope, and Raman analysis, the flame-retardancy mode of the CSV−DOPO-modified EP was deduced. First, DOPO decomposes P-containing compounds during combustion, and further releases PO and PO2 free radicals on the surface of the thermosetting resin during combustion. These free radicals capture and maintain the burning OH and H free radicals, resulting in a quenching effect. In addition, the addition of CSV−DOPO makes the EP composite produce a continuous dense carbon layer after combustion, effectively isolating the material exchange and heat conduction inside and outside of the matrix [33].

4. Conclusions

In this work, highly efficient flame-retardant EP composites were prepared by introducing CSV and DOPO into the EP. When the total addition was 3 wt%, the modified EP had a UL-94 V0 rating with an LOI of up to 30.5%, and demonstrated good flame-retardant properties. The TGA results showed that the introduction of 3 wt% CSV−DOPO into the EP reduced the initial decomposition temperature. The results of the cone calorimeter test indicated that the release of heat was restrained significantly by CSV−DOPO. The enhanced flame retardancy can be attributed to the following mechanisms: in the gas phase, the thermal decomposition products of EP/CSV/DOPO effectively trap active free radicals, thereby exerting a radical quenching effect. In the condensed phase, the polyhydroxyl groups and aromatic rings within the CSV structure, coupled with phosphorus-containing compounds generated from DOPO decomposition, facilitate the formation of a more uniform and dense carbon layer, thereby enhancing the thermal and oxygen barrier properties. The improvement in thermal stability is because, when the amino groups in CSV that are not part of the Schiff base structure react with the epoxy groups in the EP, this improves the carbonization of the matrix. In summary, this study presents a rational method for preparing EP composites with flame-retardant, smoke-suppressive, and environmentally friendly properties by using chitosan, vanillin, and DOPO as the principal raw composites.

Author Contributions

Conceptualization, Y.Y.; methodology, Y.Y.; software, Y.Y.; validation, Y.Y.; formal analysis, Y.L.; investigation, Y.L.; resources, Y.L.; data curation, Y.L.; writing—original draft preparation, Y.Y.; writing—review and editing, W.Z.; visualization, Q.K.; supervision, Q.K.; project administration, Q.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The original contributions provided in this study are included in the article. Further enquiries can be directed to the corresponding author.

Acknowledgments

This work was supported by the Natural Science of Foundation of China (Grant No. 5220421) and the Jiangsu Collaborative Innovation Center of Technology and Material of Water Treatment, China.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic diagram of the synthetic route of CSV.
Figure 1. Schematic diagram of the synthetic route of CSV.
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Figure 2. FTIR spectra of the VA, CS, and CSV.
Figure 2. FTIR spectra of the VA, CS, and CSV.
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Figure 3. (a) TGA curves of the epoxy composite under N2; (b) DTG curves of the epoxy composite under N2.
Figure 3. (a) TGA curves of the epoxy composite under N2; (b) DTG curves of the epoxy composite under N2.
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Figure 4. Video screenshots of the EP (a) and EP/3 wt% CSV−DOPO composites (b) during the UL-94 testing.
Figure 4. Video screenshots of the EP (a) and EP/3 wt% CSV−DOPO composites (b) during the UL-94 testing.
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Figure 5. Curves of the (a) HRR, (b) THR, (c) SPR, and (d) TSP of the EP composites.
Figure 5. Curves of the (a) HRR, (b) THR, (c) SPR, and (d) TSP of the EP composites.
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Figure 6. The curves of (a) CO2P; (b) mass loss of the EP composites.
Figure 6. The curves of (a) CO2P; (b) mass loss of the EP composites.
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Figure 7. Charcoal plots of the CCT dates of the EP/CSV−DOPO composites: (a) EP; (b) EP/3 wt% DOPO; (c) EP/3 wt% CSV−DOPO.
Figure 7. Charcoal plots of the CCT dates of the EP/CSV−DOPO composites: (a) EP; (b) EP/3 wt% DOPO; (c) EP/3 wt% CSV−DOPO.
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Figure 8. SEM images of (a1) the external char layer of the EP, (b1) the internal char layer of the EP, (a2) the external char layer of the EP/3 wt% DOPO composite, (b2) the internal char layer of the EP/3 wt% DOPO composite, (a3) the external char layer of the EP/3 wt% CSV−DOPO composite, and (b3) the internal char layer of the EP/3 wt% CSV−DOPO composite.
Figure 8. SEM images of (a1) the external char layer of the EP, (b1) the internal char layer of the EP, (a2) the external char layer of the EP/3 wt% DOPO composite, (b2) the internal char layer of the EP/3 wt% DOPO composite, (a3) the external char layer of the EP/3 wt% CSV−DOPO composite, and (b3) the internal char layer of the EP/3 wt% CSV−DOPO composite.
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Figure 9. Raman spectra of the char residues: (a) EP; (b) EP/3 wt% DOPO composites; (c) EP/3 wt% CSV−DOPO composites.
Figure 9. Raman spectra of the char residues: (a) EP; (b) EP/3 wt% DOPO composites; (c) EP/3 wt% CSV−DOPO composites.
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Figure 10. Mechanism diagram of the flame retarding of the EP composites by CSV−DOPO.
Figure 10. Mechanism diagram of the flame retarding of the EP composites by CSV−DOPO.
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Table 1. Components of the EP composites.
Table 1. Components of the EP composites.
CompositesComponentsP Content
EP + DDM (%)CSV−DOPO (%)DOPO (%)(wt%)
EP100000
EP/2 wt% CSV−DOPO98200.19
EP/3 wt% CSV−DOPO97300.29
EP/4 wt% CSV−DOPO96400.38
EP/2 wt% DOPO98020.29
EP/3 wt% DOPO97030.43
EP/4 wt% DOPO96040.57
Table 2. TG data of the EP/CSV-DOPOCSV−DOPO composites.
Table 2. TG data of the EP/CSV-DOPOCSV−DOPO composites.
CompositeT5%
(°C)		Tmax
(°C)		Mass Loss Rate at Tmax (wt%/min)Residues at 700 °C (%)
EP370.1390.514.817.0
EP/3 wt% CSV−DOPO351.7381.512.326.5
EP/4 wt% CSV−DOPO351.0378.413.827.1
EP/3 wt% DOPO350.8377.213.920.8
Table 3. UL-94 rating and LOI value of the EP and EP composites.
Table 3. UL-94 rating and LOI value of the EP and EP composites.
CompositesFlame Retardant
UL-94LOI (vol%)
T1 (s)T2 (s)T1 + T2 (s)Rating
EP69.332.2>50.0NR24.3
EP/2 wt% CSV−DOPO17.712.229.9V129.0
EP/3 wt% CSV−DOPO6.03.99.9V030.5
EP/4 wt% CSV−DOPO3.83.67.4V031.1
EP/2 wt% DOPO14.77.622.3V128.8
EP/3 wt% DOPO9.64.914.5V130.2
EP/4 wt% DOPO5.13.78.8V030.5
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Yang, Y.; Lu, Y.; Zhan, W.; Kong, Q. Design of a Novel Chitosan Derivatives and DOPO Flame Retardant and Its Application in Epoxy Resin. Macromol 2025, 5, 9. https://doi.org/10.3390/macromol5010009

AMA Style

Yang Y, Lu Y, Zhan W, Kong Q. Design of a Novel Chitosan Derivatives and DOPO Flame Retardant and Its Application in Epoxy Resin. Macromol. 2025; 5(1):9. https://doi.org/10.3390/macromol5010009

Chicago/Turabian Style

Yang, Yicheng, Yue Lu, Wang Zhan, and Qinghong Kong. 2025. "Design of a Novel Chitosan Derivatives and DOPO Flame Retardant and Its Application in Epoxy Resin" Macromol 5, no. 1: 9. https://doi.org/10.3390/macromol5010009

APA Style

Yang, Y., Lu, Y., Zhan, W., & Kong, Q. (2025). Design of a Novel Chitosan Derivatives and DOPO Flame Retardant and Its Application in Epoxy Resin. Macromol, 5(1), 9. https://doi.org/10.3390/macromol5010009

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