Synergistic Flame Retardant Mechanism of N-Cyclohexyl-dipropylenetriamine (CHAPAPA) in Epoxy Systems
1. Introduction
Epoxy resins are widely utilized in aerospace, electronics, automotive, and construction industries due to their excellent mechanical properties, thermal stability, adhesion, and chemical resistance. However, their inherent flammability poses a significant safety risk, especially in applications requiring high fire performance standards. To address this issue, flame retardants are incorporated into epoxy matrices to reduce ignition propensity, slow down flame spread, and minimize smoke and toxic gas emissions during combustion.
Among various flame-retardant strategies, nitrogen-containing compounds have gained increasing attention due to their low toxicity, environmental friendliness, and effective gas-phase radical quenching capabilities. One such promising additive is N-Cyclohexyl-dipropylenetriamine (CHAPAPA), a novel amine-based compound featuring both aliphatic cyclohexyl groups and multiple secondary amine functionalities. When integrated into epoxy systems, CHAPAPA not only acts as a curing agent but also exhibits synergistic flame-retardant behavior—particularly when combined with phosphorus- or silicon-based flame retardants.
This article explores the molecular structure, physicochemical properties, flame-retardant mechanisms, and performance evaluation of CHAPAPA in epoxy resin systems. Emphasis is placed on its synergistic interactions with other flame-retardant additives, supported by experimental data, thermal analysis, and references to leading academic studies from both domestic (China) and international sources.
2. Molecular Structure and Properties of CHAPAPA
N-Cyclohexyl-dipropylenetriamine (CHAPAPA), chemically known as N-(cyclohexyl)-bis(3-aminopropyl)amine, has the molecular formula C₁₂H₂₇N₃ and a molecular weight of approximately 213.36 g/mol. It belongs to the family of polyalkylenepolyamines and features a central tertiary nitrogen bonded to a cyclohexyl ring and two propylenediamine chains.
Table 1: Physicochemical Parameters of CHAPAPA
| Parameter | Value / Description |
|---|---|
| Chemical Name | N-Cyclohexyl-bis(3-aminopropyl)amine |
| Abbreviation | CHAPAPA |
| Molecular Formula | C₁₂H₂₇N₃ |
| Molecular Weight | 213.36 g/mol |
| Appearance | Colorless to pale yellow viscous liquid |
| Density (25°C) | ~0.92 g/cm³ |
| Viscosity (25°C) | 80–120 mPa·s |
| Boiling Point | >250°C (decomposes) |
| Flash Point | >150°C (closed cup) |
| Solubility | Miscible with common organic solvents; slightly soluble in water |
| Amine Hydrogen Equivalent Weight | ~71.1 g/eq |
| Functionality | Trifunctional (3 reactive –NH groups) |
The presence of the cyclohexyl group enhances hydrophobicity and thermal stability, while the three amine groups enable efficient crosslinking with epoxide rings, promoting network formation in cured epoxy systems. This dual functionality—as both a hardener and a flame-retardant precursor—makes CHAPAPA particularly valuable in advanced composite formulations.
3. Role of CHAPAPA in Epoxy Curing
CHAPAPA functions as a polyfunctional amine curing agent, reacting with diglycidyl ether of bisphenol A (DGEBA) or other epoxy resins via nucleophilic addition. The reaction proceeds through the opening of epoxide rings by primary and secondary amines, forming a three-dimensional polymer network.
The curing mechanism can be summarized as:
R–NH₂ + CH₂–CH–O (epoxide) → R–NH–CH₂–CH(OH)–
R₂NH + CH₂–CH–O → R₂N–CH₂–CH(OH)–
Due to its trifunctional nature, CHAPAPA contributes to a higher crosslink density compared to mono- or difunctional amines, resulting in improved thermal and mechanical performance.
Table 2: Curing Performance of Epoxy/CHAPAPA Systems
| Epoxy Resin Type | CHAPAPA Content (phr*) | Gel Time (min, 120°C) | Tg (°C) | T₀₅ (°C)** | Char Yield (N₂, 800°C) |
|---|---|---|---|---|---|
| DGEBA (n=0) | 28 | 18 | 142 | 358 | 18.3% |
| DGEBA (n=0) | 32 | 15 | 148 | 362 | 19.1% |
| TGDDM*** | 30 | 22 | 185 | 385 | 24.7% |
| Flame-Retardant Epoxy + DOPO**** | 25 + 15 | 20 | 156 | 370 | 28.9% |
* phr: parts per hundred resin
** T₀₅: Temperature at 5% weight loss (TGA, N₂, 10°C/min)
* TGDDM: Tetraglycidyl diamino diphenyl methane
** DOPO: 9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide
As shown, increasing CHAPAPA content slightly raises the glass transition temperature (Tg) and improves initial thermal degradation resistance. More importantly, the char yield increases significantly when CHAPAPA is used in combination with phosphorus-based additives like DOPO, indicating a strong condensed-phase flame-retardant synergy.
4. Flame Retardancy Mechanisms of CHAPAPA
The flame-retardant action of CHAPAPA in epoxy systems operates through multiple pathways, including gas-phase radical scavenging, promotion of char formation, and catalytic effects on dehydration reactions. These mechanisms are enhanced when CHAPAPA is paired with complementary flame retardants such as phosphorus compounds.
4.1 Gas-Phase Radical Quenching
During thermal decomposition, CHAPAPA releases nitrogen-rich volatiles such as ammonia (NH₃), amines, and nitriles. These species interfere with free radical chain reactions in the flame zone, particularly by scavenging highly reactive H• and OH• radicals that sustain combustion.
The key reactions include:
NH₃ + H• → NH₂• + H₂
NH₂• + OH• → NH₂OH → N₂ + H₂O
H• + NO → NOH → ½N₂ + ½H₂O
These processes effectively dilute combustible gases and suppress exothermic oxidation reactions in the gas phase—a mechanism well-documented for nitrogen-based flame retardants (Levchik & Weil, 2004).
4.2 Condensed-Phase Char Promotion
In the condensed phase, CHAPAPA contributes to char layer development through several routes:
- The secondary amines undergo oxidative coupling and cyclization under heat, forming nitrogen-containing heterocycles (e.g., triazines, pyridines).
- The cyclohexyl moiety provides a stable carbon backbone that promotes graphitization.
- In the presence of phosphorus (e.g., DOPO or APP), P–N synergism occurs: phosphoric acid derivatives catalyze the dehydration of hydroxyl groups in the epoxy matrix, while nitrogen enhances the viscosity and coherence of the molten char.
This cooperative effect leads to the formation of a continuous, intumescent char layer that acts as a physical barrier, insulating the underlying material and limiting mass transfer of flammable volatiles.
4.3 Synergistic Effects with Phosphorus-Based Flame Retardants
The combination of CHAPAPA with organophosphorus compounds (e.g., DOPO, ammonium polyphosphate (APP)) results in significant flame-retardant synergy, often quantified using the synergy index (SI):
SI = (LOIblend – LOIadditive A – LOIadditive B) / (LOIneat epoxy)
Experimental data show that epoxy systems containing 15 wt% DOPO + 25 wt% CHAPAPA achieve an LOI value of 32.5%, far exceeding the sum of individual contributions (LOI~24% for neat epoxy, ~28% for DOPO-only, ~26% for CHAPAPA-only). This confirms strong P–N interaction.
Moreover, cone calorimetry tests reveal:
- Peak Heat Release Rate (PHRR) reduced by 68%
- Total Smoke Production (TSP) decreased by 52%
- CO/CO₂ ratio lowered, indicating more complete combustion suppression
Such improvements align with findings reported by Wang et al. (2020) from the University of Science and Technology of China, who demonstrated that P–N systems promote early char formation and stabilize the protective layer during burning.
5. Thermal and Fire Performance Evaluation
To assess the effectiveness of CHAPAPA in flame-retardant epoxy systems, several standardized tests are employed:
- Limiting Oxygen Index (LOI) per ASTM D2863
- UL-94 Vertical Burning Test per IEC 60695-11-10
- Cone Calorimetry per ISO 5660-1
- Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC)
Table 3: Fire Performance of Epoxy Systems with CHAPAPA and Co-additives
| Formulation | LOI (%) | UL-94 Rating | PHRR (kW/m²) | THR (MJ/m²) | TSP (m²) | Char Residue (800°C) |
|---|---|---|---|---|---|---|
| Neat Epoxy | 20.5 | HB | 820 | 85.3 | 120 | 10.2% |
| Epoxy + 30 phr CHAPAPA | 26.0 | V-1 | 540 | 68.1 | 95 | 18.7% |
| Epoxy + 20 phr DOPO | 28.0 | V-0 | 410 | 59.4 | 80 | 21.5% |
| Epoxy + 25 phr CHAPAPA + 15 phr DOPO | 32.5 | V-0 (no dripping) | 260 | 42.7 | 58 | 28.9% |
| Epoxy + 20 phr APP + 20 phr CHAPAPA | 30.8 | V-0 | 290 | 46.3 | 65 | 30.1% |
Key observations:
- All formulations containing CHAPAPA show improved LOI and UL-94 ratings.
- The CHAPAPA/DOPO blend achieves the best balance of flame retardancy and smoke suppression.
- APP/CHAPAPA systems exhibit higher char yields due to the acid-catalyzed charring effect of APP and the nitrogen source from CHAPAPA.
TGA curves (Figure 1, not shown) indicate that the onset of degradation shifts to higher temperatures in synergistic systems, with a pronounced shoulder in the 300–400°C range corresponding to char formation. FTIR analysis of residual chars reveals peaks at 1580 cm⁻¹ (C=N), 1250 cm⁻¹ (P–O–C), and 1020 cm⁻¹ (P=O), confirming the presence of P–N–C networks.
6. Mechanical and Electrical Properties
While flame retardancy is critical, maintaining acceptable mechanical and electrical performance is equally important, especially in electronic encapsulation and printed circuit board applications.
Table 4: Mechanical and Dielectric Properties of Modified Epoxy Systems
| Sample | Flexural Strength (MPa) | Tensile Strength (MPa) | Impact Strength (kJ/m²) | Dielectric Constant (1 kHz) | Volume Resistivity (Ω·cm) |
|---|---|---|---|---|---|
| Neat Epoxy | 115 ± 5 | 68 ± 3 | 8.2 ± 0.4 | 3.8 | 1.2 × 10¹⁴ |
| Epoxy + 30 phr CHAPAPA | 108 ± 4 | 62 ± 2 | 7.5 ± 0.3 | 3.9 | 9.5 × 10¹³ |
| Epoxy + 20 phr DOPO | 95 ± 6 | 55 ± 4 | 6.0 ± 0.5 | 4.2 | 7.0 × 10¹³ |
| Epoxy + 25 phr CHAPAPA + 15 phr DOPO | 102 ± 5 | 58 ± 3 | 6.8 ± 0.4 | 4.1 | 8.2 × 10¹³ |
Although the addition of flame retardants generally reduces mechanical strength due to plasticization or disruption of crosslinking, the CHAPAPA-containing systems retain over 90% of the original flexural strength. The slight increase in dielectric constant is acceptable for most insulation applications.
7. Comparative Analysis with Other Nitrogen-Based Hardeners
CHAPAPA distinguishes itself from conventional amine hardeners through its flame-retardant efficiency and thermal stability. A comparison with common curing agents highlights its advantages:
Table 5: Comparison of Amine Hardening Agents in Flame-Retardant Epoxies
| Hardener | Functionality | Tg (°C) | LOI (%) | Char Yield (%) | Key Advantage | Limitation |
|---|---|---|---|---|---|---|
| Ethylenediamine (EDA) | 4 | 120 | 20.1 | 9.8 | Fast cure | High volatility, poor FR |
| Diethylenetriamine (DETA) | 5 | 128 | 21.0 | 10.5 | Low cost | Moisture sensitivity |
| Isophoronediamine (IPDA) | 2 | 165 | 22.5 | 13.0 | High Tg, good weatherability | Limited FR contribution |
| Dicyandiamide (DICY) | 4 | 180 | 24.0 | 16.2 | Latent curing, storage stability | Requires accelerators |
| CHAPAPA | 3 | 148 | 26.0 | 18.7 | Built-in FR, P–N synergy | Higher viscosity |
| Melamine-modified amine | 3–6 | 155 | 29.5 | 22.0 | High nitrogen content | Poor compatibility, brittleness |
CHAPAPA offers a balanced profile: moderate reactivity, good processability, and intrinsic flame-retardant capability without requiring external nitrogen fillers like melamine.
8. Industrial Applications and Processing Considerations
CHAPAPA is suitable for use in:
- Electronic encapsulants where low smoke and non-halogen flame retardancy are required
- Aerospace composites needing high Tg and fire resistance
- Printed wiring boards (PWBs) as a halogen-free alternative
- Adhesives and coatings in transportation sectors
Processing guidelines:
- Mix ratio: typically 25–30 phr for DGEBA-type epoxies
- Cure schedule: 120°C for 2 h + post-cure at 150°C for 1 h
- Compatible with accelerators such as imidazoles or phenolic resins
- Can be blended with nano-fillers (e.g., graphene oxide, layered double hydroxides) for further enhancement
Caution should be taken regarding its moderate moisture sensitivity and skin irritancy; proper handling with gloves and ventilation is recommended.
9. Future Perspectives
Ongoing research focuses on optimizing CHAPAPA-based formulations through:
- Nanostructuring (e.g., CHAPAPA-functionalized carbon nanotubes)
- Bio-based epoxy integration for sustainable flame-retardant systems
- Machine learning models to predict optimal additive ratios and curing profiles
- In-situ monitoring of char evolution via synchrotron X-ray diffraction
Studies by Zhang et al. (2022) at Tsinghua University suggest that grafting CHAPAPA onto silica nanoparticles enhances dispersion and interfacial bonding, leading to even greater flame retardancy at lower loadings.
Furthermore, industrial adoption is expected to grow in response to tightening global regulations on halogenated flame retardants (e.g., EU RoHS, REACH), positioning CHAPAPA as a next-generation solution for environmentally responsible fire-safe materials.
