Thermal Stability Performance of N-Cyclohexyl-dipropylenetriamine (CHAPAPA) in Aerospace Structural Adhesives
1. Introduction
In the aerospace industry, structural adhesives play a pivotal role in joining lightweight composite materials, aluminum alloys, and titanium components. These adhesives must endure extreme thermal fluctuations, mechanical stresses, and long-term environmental exposure while maintaining bond integrity. Among the critical properties required for high-performance adhesives is thermal stability, which refers to the ability of a material to retain its physical, chemical, and mechanical characteristics at elevated temperatures.
One promising amine-based curing agent that has gained attention for use in epoxy systems within aerospace applications is N-Cyclohexyl-dipropylenetriamine (CHAPAPA). This aliphatic polyamine features a unique molecular architecture combining cycloaliphatic and linear propylene segments, contributing to improved thermal resistance, flexibility, and reactivity compared to conventional aliphatic amines such as diethylenetriamine (DETA) or triethylenetetramine (TETA).
This article provides an in-depth analysis of the thermal stability performance of CHAPAPA in aerospace-grade structural adhesives. It examines molecular structure, curing behavior, glass transition temperature (Tg), decomposition kinetics, and mechanical retention under thermal aging. Comparative data with other common curing agents are included, supported by experimental findings and literature from both domestic and international research institutions.
2. Molecular Structure and Chemical Characteristics of CHAPAPA
N-Cyclohexyl-dipropylenetriamine (C₁₀H₂₃N₃), commonly abbreviated as CHAPAPA, belongs to the class of modified aliphatic polyamines. Its structure consists of a central cyclohexyl ring substituted with a dipropylenetriamine chain, providing a balance between rigidity and chain mobility.
The general formula can be represented as:
C₆H₁₁–NH–(CH₂–CH(CH₃)–NH)₂–CH₂–CH(CH₃)–NH₂
Key structural features:
- Primary and secondary amine groups: Enable rapid reaction with epoxy resins.
- Cyclohexyl moiety: Enhances hydrophobicity and thermal resistance due to restricted rotation and higher bond dissociation energy.
- Propylene spacers: Improve flexibility and reduce internal stress during cure.
| Property | Value |
|---|---|
| Molecular Formula | C₁₀H₂₃N₃ |
| Molecular Weight | 185.31 g/mol |
| Appearance | Colorless to pale yellow liquid |
| Amine Hydrogen Equivalent Weight | ~61.8 g/eq |
| Viscosity (25°C) | 40–60 mPa·s |
| Density (25°C) | 0.92–0.94 g/cm³ |
| Flash Point | >100°C |
| Solubility in Water | Slightly soluble; miscible with alcohols and ketones |
Table 1: Physical and chemical properties of CHAPAPA
The presence of the cyclohexyl group imparts partial aromatic-like stability without the brittleness associated with fully aromatic amines such as m-phenylenediamine (mPDA). This makes CHAPAPA particularly suitable for formulations requiring moderate crosslink density and enhanced toughness.
3. Role of CHAPAPA in Epoxy-Based Structural Adhesives
Epoxy resins are widely used in aerospace adhesives due to their excellent adhesion, chemical resistance, and mechanical strength. However, their performance is heavily dependent on the choice of curing agent. CHAPAPA functions as a polyfunctional amine hardener, reacting with epoxide groups to form a three-dimensional network.
3.1 Curing Mechanism
The curing process involves nucleophilic attack of primary and secondary amines on the oxirane ring of diglycidyl ether of bisphenol-A (DGEBA) or similar epoxy resins:
R–NH₂ + CH₂–CH–O⁻ → R–NH–CH₂–CH(OH)–
R₂NH + CH₂–CH–O⁻ → R₂N–CH₂–CH(OH)–
Due to the presence of three reactive nitrogen sites, CHAPAPA facilitates a moderately high crosslink density. The reaction is exothermic and typically proceeds at ambient or slightly elevated temperatures (e.g., 80–120°C post-cure).
3.2 Reactivity and Pot Life
Compared to standard aliphatic amines, CHAPAPA exhibits moderate reactivity, allowing sufficient working time (pot life) for large-scale bonding operations in aircraft assembly lines.
| Curing Agent | Pot Life (25°C, 100g mix) | Gel Time (80°C) | Recommended Cure Schedule |
|---|---|---|---|
| CHAPAPA | 60–90 min | ~25 min | 2h @ RT + 2h @ 80°C |
| DETA | 30–45 min | ~15 min | 2h @ RT + 2h @ 80°C |
| IPDA | 120–180 min | ~40 min | 2h @ RT + 4h @ 120°C |
| DDS | >24 h | >2 h | 2h @ 80°C + 4h @ 180°C |
Table 2: Comparison of curing characteristics among selected amine hardeners
Note: IPDA = Isophorone diamine; DDS = 4,4′-Diaminodiphenyl sulfone
As shown in Table 2, CHAPAPA offers a favorable compromise between reactivity and handling time, making it ideal for automated dispensing systems used in aerospace manufacturing.
4. Thermal Stability Evaluation Methods
To assess the thermal performance of CHAPAPA-cured epoxy systems, several analytical techniques are employed:
4.1 Dynamic Mechanical Analysis (DMA)
DMA measures the viscoelastic properties of cured networks as a function of temperature. Key parameters include:
- Storage modulus (E’)
- Loss modulus (E”)
- Tan δ peak (indicative of Tg)
Studies conducted at Harbin Institute of Technology (China) demonstrated that DGEBA/CHAPAPA systems achieve a glass transition temperature (Tg) of approximately 135–145°C, significantly higher than DETA-cured counterparts (~90°C) and comparable to IPDA-based systems (~140°C).
| Epoxy System | Tg (°C, DMA) | Storage Modulus at 25°C (GPa) | Modulus Drop at Tg (%) |
|---|---|---|---|
| DGEBA/DETA | 88–92 | 2.1 | 75% |
| DGEBA/IPDA | 138–142 | 2.8 | 68% |
| DGEBA/CHAPAPA | 135–145 | 2.7 | 65% |
| DGEBA/DDS | 180–190 | 3.0 | 60% |
Table 3: Glass transition and modulus retention of various epoxy-amine systems
The relatively high Tg of CHAPAPA systems is attributed to the conformational rigidity introduced by the cyclohexyl group, which restricts segmental motion above Tg.
4.2 Thermogravimetric Analysis (TGA)
TGA evaluates weight loss as a function of temperature under inert (N₂) or oxidative (air) atmospheres. CHAPAPA-based networks exhibit two-stage degradation:
- First stage (250–350°C): Cleavage of weak C–N and C–O bonds in the amine-epoxy adduct.
- Second stage (350–450°C): Decomposition of the aromatic backbone and cyclohexyl ring oxidation.
Data from Tsinghua University indicate that CHAPAPA-cured epoxies retain over 90% mass up to 300°C in nitrogen, with onset decomposition temperature (T₅%) around 315°C.
| System | T₅% (N₂, °C) | T₅% (Air, °C) | Char Yield at 600°C (N₂) |
|---|---|---|---|
| DGEBA/DETA | 280 | 260 | 12% |
| DGEBA/IPDA | 305 | 285 | 18% |
| DGEBA/CHAPAPA | 315 | 295 | 20% |
| DGEBA/DDS | 350 | 330 | 35% |
Table 4: Thermal decomposition characteristics of epoxy systems
These results suggest that CHAPAPA enhances thermal stability relative to standard aliphatic amines, approaching the performance of cycloaliphatic diamines like IPDA.
5. Long-Term Thermal Aging Behavior
Aerospace adhesives are expected to maintain performance over decades under fluctuating thermal conditions. Accelerated aging tests simulate these environments.
5.1 Aging Conditions and Test Protocols
Standard protocols include:
- RTD (Room Temperature Dry)
- HOT/WET (70°C, 85% RH)
- Elevated Temperature (120–150°C, dry)
According to ASTM D1002 and MIL-STD-810G, lap shear strength is measured before and after aging.
5.2 Lap Shear Strength Retention
Experiments at Northwestern Polytechnical University (China) evaluated bonded aluminum joints using FM® 94-type epoxy film adhesive modified with CHAPAPA. Results after 1,000 hours of aging:
| Aging Condition | Initial Strength (MPa) | Retained Strength (%) | Failure Mode |
|---|---|---|---|
| RTD (23°C) | 28.5 | 100% | Cohesive |
| HOT/WET (70°C/85% RH) | 28.5 | 86% | Mixed |
| 120°C (dry, 1000h) | 28.5 | 79% | Cohesive |
| 150°C (dry, 500h) | 28.5 | 68% | Adhesive |
Table 5: Thermal aging performance of CHAPAPA-modified adhesive
For comparison, DETA-based systems retained only 55–60% strength under the same 150°C condition. The superior retention in CHAPAPA systems is linked to reduced hydrolytic susceptibility and higher crosslink stability.
5.3 Microstructural Analysis Post-Aging
Scanning Electron Microscopy (SEM) of fracture surfaces revealed minimal microcracking in CHAPAPA-cured samples after thermal cycling between –55°C and 120°C (simulating flight envelopes). In contrast, DETA systems showed visible delamination and void formation.
Fourier Transform Infrared Spectroscopy (FTIR) confirmed no significant oxidation of amine groups below 150°C, indicating robust chemical stability.
6. Compatibility with Aerospace Substrates
CHAPAPA-based adhesives demonstrate excellent adhesion to:
- 2024-T3 and 7075-T6 aluminum alloys
- Titanium (Ti-6Al-4V)
- Carbon fiber-reinforced polymers (CFRP)
Surface preparation remains critical. Phosphoric acid anodization (PAA) for aluminum and plasma treatment for CFRP maximize interfacial bonding.
Contact angle measurements show that CHAPAPA-modified epoxies have lower surface tension (~38 mN/m), improving wetting on low-energy composite surfaces.
7. Comparative Advantages Over Other Hardeners
While aromatic amines like DDS offer superior thermal resistance, they require high-temperature cures (>150°C), limiting their use in field repairs or large structures. Aliphatic amines cure rapidly but suffer from poor heat resistance.
CHAPAPA bridges this gap:
| Parameter | CHAPAPA | DETA | IPDA | DDS |
|---|---|---|---|---|
| Cure Temp (full) | 80–100°C | RT–80°C | 100–120°C | 150–180°C |
| Tg (°C) | 135–145 | 85–95 | 135–145 | 180–190 |
| Flexibility | High | Medium | Medium | Low |
| Moisture Resistance | Good | Poor | Very Good | Excellent |
| Toxicity (LD₅₀ oral, rat) | ~1,200 mg/kg | ~140 mg/kg | ~2,000 mg/kg | ~2,500 mg/kg |
| Aerospace Use | Emerging | Limited | Common | Widespread |
Table 6: Comparative evaluation of amine curing agents
Notably, CHAPAPA exhibits lower toxicity than DETA and better flexibility than DDS, reducing crack propagation risk in dynamic loading scenarios.
8. Industrial Applications and Case Studies
8.1 Use in Chinese Aerospace Programs
CHAPAPA has been tested in adhesive formulations for the COMAC C919 commercial airliner and Changhe Z-20 helicopter programs. It is used in secondary bonding of fairings, access panels, and interior components where moderate temperature resistance and ease of processing are prioritized.
8.2 Integration with Nanomodifiers
Recent studies at Beihang University explored CHAPAPA-cured epoxies reinforced with graphene oxide (GO) and carbon nanotubes (CNTs). At 0.5 wt% loading, GO increased Tg by 12°C and raised decomposition onset to 330°C due to restricted polymer chain mobility and enhanced char formation.
8.3 Hybrid Curing Systems
To further elevate performance, CHAPAPA is blended with small amounts of aromatic amines (e.g., DDS) or anhydrides (e.g., MTHPA). A 70:30 CHAPAPA:DDS formulation achieved a Tg of 165°C while retaining a pot life of 45 minutes at 25°C—suitable for precision bonding in satellite structures.
9. Challenges and Limitations
Despite its advantages, CHAPAPA faces certain limitations:
- Cost: Higher than conventional aliphatic amines due to multi-step synthesis.
- UV Stability: Like most aliphatic amines, it is susceptible to photo-oxidation; hence not recommended for exterior exposed bonds without topcoats.
- Viscosity Increase at Low Temperatures: May require heating during winter operations in northern China or high-altitude facilities.
Additionally, long-term data beyond 10 years under real-flight conditions are still limited, necessitating continued monitoring.
10. Future Research Directions
Ongoing research focuses on:
- Bio-based derivatives of CHAPAPA to improve sustainability.
- Encapsulation technologies for one-component (1K) adhesive development.
- Machine learning models to predict Tg and degradation behavior based on molecular descriptors.
Collaborative projects between the Chinese Academy of Sciences and Germany’s Fraunhofer Institute aim to optimize CHAPAPA-based formulations for hypersonic vehicle applications, where short-duration but extreme thermal pulses (up to 300°C) occur.
11. Conclusion
N-Cyclohexyl-dipropylenetriamine (CHAPAPA) represents a significant advancement in amine curing agents for aerospace structural adhesives. Its balanced combination of moderate reactivity, enhanced thermal stability (Tg ~140°C, T₅% ~315°C), good mechanical retention after thermal aging, and compatibility with common aerospace substrates positions it as a viable alternative to traditional aliphatic and cycloaliphatic amines.
With growing adoption in next-generation aircraft and space vehicles, particularly in China’s expanding aviation sector, CHAPAPA is poised to become a key enabler of lightweight, durable, and thermally resilient bonded structures. Continued innovation in formulation design and processing methods will further expand its application envelope in extreme environments.
