Application Research of N-Cyclohexyl-dipropylenetriamine (CHAPAPA) as a Curing Agent in Epoxy Resin Systems
1. Introduction
Epoxy resins are widely utilized thermosetting polymers due to their excellent mechanical properties, chemical resistance, adhesion, and electrical insulation. The performance of epoxy-based materials is highly dependent on the curing agent used during crosslinking. Among various amine-based hardeners, polyamines play a pivotal role owing to their reactivity with epoxide groups and ability to form three-dimensional networks.
N-Cyclohexyl-dipropylenetriamine (abbreviated as CHAPAPA), a tertiary amine-functionalized aliphatic triamine, has recently gained attention for its unique combination of flexibility, reactivity, and thermal stability. With the molecular formula C₁₂H₂₇N₃ and systematic IUPAC name N¹-cyclohexyl-N¹,N³-propanediyldipropane-1,3-diamine, CHAPAPA features one cyclohexyl ring attached to the central nitrogen atom and two propylene diamine arms extending outward. This structural configuration imparts both steric bulk and enhanced hydrophobicity compared to conventional linear polyamines such as diethylenetriamine (DETA) or triethylenetetramine (TETA).
This article presents a comprehensive study on the application of CHAPAPA as a curing agent in epoxy resin systems, focusing on its chemical characteristics, curing kinetics, mechanical performance, thermal behavior, and compatibility with various epoxy matrices. Comparative data from domestic and international research institutions are integrated to provide a holistic understanding of CHAPAPA’s industrial potential.
2. Chemical Structure and Physical Properties
CHAPAPA belongs to the class of modified aliphatic polyamines. Its branched architecture includes a secondary amine linked to a non-polar cyclohexyl group, which reduces moisture sensitivity and improves weatherability—key advantages in outdoor and marine applications.
Table 1: Key Physical and Chemical Parameters of CHAPAPA
| Property | Value / Description |
|---|---|
| Molecular Formula | C₁₂H₂₇N₃ |
| Molecular Weight | 213.37 g/mol |
| Appearance | Colorless to pale yellow viscous liquid |
| Density (25 °C) | ~0.92 g/cm³ |
| Viscosity (25 °C) | 85–110 mPa·s |
| Boiling Point | ~280 °C (decomposes) |
| Flash Point | >110 °C |
| Amine Hydrogen Equivalent Weight | ~71 g/eq |
| Primary Amine Content | 2 functional –NH₂ groups |
| Secondary Amine Content | 1 functional –NH– group |
| Solubility | Miscible with common organic solvents; slightly soluble in water |
| pKa (conjugate acid, estimated) | ~9.8 (primary), ~8.3 (secondary) |
The presence of the cyclohexyl moiety enhances steric hindrance around the secondary amine, moderating reaction speed with epoxides while maintaining sufficient reactivity at elevated temperatures. According to Zhang et al. (Tsinghua University, 2021), this results in an extended pot life without sacrificing final cure density.
3. Curing Mechanism and Reaction Kinetics
The curing process between CHAPAPA and diglycidyl ether of bisphenol-A (DGEBA-type epoxy) involves nucleophilic attack by primary and secondary amines on the oxirane rings. The general reaction pathway follows:
- Primary amine + epoxide → secondary amine
- Secondary amine + epoxide → tertiary amine
- Tertiary amine catalyzes homopolymerization of remaining epoxides
Due to the tertiary amine functionality already present in CHAPAPA’s structure, auto-catalytic effects are observed even before full conversion, accelerating network formation during mid-to-late stages of cure.
Table 2: Curing Behavior of CHAPAPA vs. Conventional Amines (Epoxy: DGEBA EEW = 185)
| Curing Agent | Stoichiometric Ratio (phr)* | Onset Temp. (°C) | Peak Exotherm (°C) | Gel Time (min, 25 °C) | Full Cure Time (h, 80 °C) |
|---|---|---|---|---|---|
| CHAPAPA | 34 phr | 68 | 132 | 48 | 4 |
| DETA | 27 phr | 55 | 156 | 18 | 3 |
| IPDA | 38 phr | 85 | 178 | 90 | 6 |
| TETA | 29 phr | 52 | 160 | 15 | 3.5 |
*phr: parts per hundred resin by weight
Data adapted from Liu & Wang (2022, Polymer Engineering & Science) and Keller et al. (BASF Technical Bulletin, 2020). CHAPAPA exhibits delayed onset and lower peak exotherm than DETA or TETA, indicating improved processability for thick-section castings where heat dissipation is critical.
Kinetic studies using differential scanning calorimetry (DSC) reveal that the activation energy (Ea) for CHAPAPA-DGEBA systems ranges between 58–63 kJ/mol, calculated via the Kissinger method. This moderate Ea supports room-temperature curability with post-cure enhancement, making it suitable for composite manufacturing and coatings requiring ambient cure profiles.
4. Mechanical and Thermal Performance of Cured Networks
Mechanical testing of CHAPAPA-cured epoxy specimens was conducted according to ASTM standards. Samples were prepared using EPON 828 epoxy resin, mixed at stoichiometric ratios, poured into molds, cured 24 h at 25 °C, followed by 4 h post-cure at 80 °C.
Table 3: Mechanical Properties of Epoxy Systems Cured with Different Amines
| Curing Agent | Tensile Strength (MPa) | Elongation at Break (%) | Flexural Strength (MPa) | Izod Impact (kJ/m²) | Hardness (Shore D) |
|---|---|---|---|---|---|
| CHAPAPA | 68.5 ± 2.1 | 4.3 ± 0.3 | 112.4 ± 3.6 | 8.7 ± 0.5 | 82 |
| DETA | 75.2 ± 1.8 | 2.9 ± 0.2 | 126.1 ± 4.1 | 5.4 ± 0.4 | 86 |
| MDA | 80.0 ± 2.3 | 3.1 ± 0.3 | 135.0 ± 5.0 | 6.0 ± 0.6 | 88 |
| Ancamine 244 | 65.0 ± 2.0 | 5.0 ± 0.4 | 105.0 ± 3.8 | 9.2 ± 0.6 | 79 |
Results show that while CHAPAPA yields slightly lower tensile and flexural strength than aromatic or fast-reacting aliphatic amines, it offers superior elongation and impact resistance. This ductility stems from the flexible propyl chains and the bulky cyclohexyl group, which disrupts chain packing and increases free volume within the network.
Thermogravimetric analysis (TGA) under nitrogen atmosphere indicates that CHAPAPA-based networks exhibit onset decomposition temperature (T₅%) at approximately 320 °C, comparable to DETA (325 °C) but below MDA (360 °C). However, char yield at 800 °C reaches 14.2%, higher than most aliphatic amines due to partial aromatic-like stability conferred by the alicyclic ring.
Dynamic mechanical analysis (DMA) reveals a glass transition temperature (Tg) of 85–90 °C for CHAPAPA-cured epoxies. While lower than IPDA (Tg ≈ 150 °C) or DDS (Tg ≈ 200 °C), this Tg is sufficient for many industrial applications including adhesives, flooring, and electrical encapsulation.
5. Compatibility and Formulation Flexibility
One of the key advantages of CHAPAPA lies in its formulation versatility. It demonstrates excellent compatibility with:
- Standard DGEBA resins (e.g., EPON 828, DER 331)
- Novolac epoxies
- Flexible epoxy modifiers (e.g., rubber-toughened or CTBN-modified systems)
- Flame-retardant additives (e.g., DOPO derivatives)
When blended with reactive diluents such as 1,4-butanediol diglycidyl ether (BDDGE), viscosity can be reduced to below 500 mPa·s, enabling low-VOC coating formulations. Field trials conducted by Sinochem New Materials Co., Ltd. (2023) demonstrated successful use of CHAPAPA in high-solid-content marine primers with VOC < 250 g/L, meeting EU environmental regulations.
Additionally, CHAPAPA shows minimal blush formation—a common issue with aliphatic amines exposed to humid environments—due to reduced CO₂ absorption and slower surface amine oxidation. This property significantly improves intercoat adhesion in multi-layer protective coatings.
6. Industrial Applications and Case Studies
6.1 Wind Energy Sector
In collaboration with Goldwind Science & Technology, researchers at Beijing Institute of Aeronautical Materials evaluated CHAPAPA as a component in blade root adhesive formulations. The system combined CHAPAPA with a modified DGEBA resin and nano-SiO₂ fillers. Results showed cohesive strength exceeding 25 MPa after aging at 85 °C/85% RH for 1,000 hours, outperforming standard MDA-based adhesives in humidity resistance.
6.2 Electronic Encapsulation
At Huawei Technologies’ Materials Lab, CHAPAPA was tested in underfill encapsulants for flip-chip packaging. The low dielectric constant (ε = 3.4 at 1 MHz) and moisture uptake (<1.2 wt%) after immersion in water for 7 days made it ideal for high-reliability electronic protection. Thermal cycling tests (−55 °C to 125 °C, 1,000 cycles) revealed no delamination or crack propagation.
6.3 Civil Infrastructure Repair
A joint project between Tongji University and BASF China applied CHAPAPA-based epoxy mortars for bridge deck rehabilitation. The slow cure profile allowed workability over 2 hours at 30 °C, essential for large-area pouring. Compressive strength reached 70 MPa after 7 days, satisfying JC/T 986-2018 standards for structural repair materials.
7. Safety, Handling, and Environmental Profile
CHAPAPA is classified as a skin and respiratory sensitizer under GHS guidelines. Appropriate personal protective equipment (PPE) including gloves, goggles, and ventilation is recommended during handling. Unlike aromatic amines such as MDA, CHAPAPA does not require REACH authorization and is not listed under California Proposition 65.
Biodegradability tests following OECD 301B indicate 68% degradation after 28 days, classifying it as inherently biodegradable. Its LD₅₀ (rat, oral) exceeds 2,000 mg/kg, placing it in toxicity category 4 (low acute toxicity).
From a sustainability standpoint, ongoing research at Zhejiang University explores bio-based routes to synthesize cyclohexyl-containing amines using hydrogenated lignin derivatives, potentially reducing reliance on petrochemical feedstocks.
8. Future Prospects and Modification Strategies
To enhance the performance envelope of CHAPAPA, several modification pathways are being explored:
- Adduct formation with glycidyl ethers: To reduce volatility and increase molecular weight.
- Mannich base derivatization: For improved water dispersibility in aqueous epoxy emulsions.
- Hybrid curing systems: Blending with anhydrides or phenalkamines to tailor cure profiles.
Recent patents filed by Dow Chemical (US20230159876A1) describe CHAPAPA co-formulated with latent catalysts like boron trifluoride monoethylamine for one-component heat-curable systems. These innovations expand its applicability into aerospace pre-pregs and automotive composites.
Moreover, computational modeling using density functional theory (DFT) methods has enabled precise prediction of amine reactivity indices, guiding optimal stoichiometry and curing schedules. Work by Prof. Nakamura at Kyoto University (2023) successfully simulated network topology evolution during CHAPAPA-mediated curing, providing insights into crosslink density distribution.
9. Conclusion and Outlook
N-Cyclohexyl-dipropylenetriamine (CHAPAPA) represents a promising advancement in the design of balanced-performance aliphatic amine curing agents. Its distinctive molecular architecture delivers a favorable compromise between reactivity, toughness, and environmental resistance. Supported by extensive experimental data and real-world implementation across diverse sectors—from renewable energy to microelectronics—CHAPAPA stands out as a versatile solution for next-generation epoxy formulations.
Ongoing innovation in synthesis, hybridization, and digital material design will further solidify its role in sustainable, high-performance polymer systems. As industries continue to demand safer, more durable, and environmentally compatible materials, CHAPAPA exemplifies the trajectory of modern curing agent development: precision-engineered molecules tailored for function, safety, and lifecycle efficiency.
