Application of N-Cyclohexyl-dipropylenetriamine (CHAPAPA) as a Curing Agent in Epoxy Resins
Introduction
Epoxy resins are widely utilized thermosetting polymers due to their excellent mechanical strength, chemical resistance, adhesion, and electrical insulation properties. These characteristics make them indispensable in industries such as aerospace, automotive, electronics, construction, and coatings. However, the performance of epoxy resins is highly dependent on the selection of an appropriate curing agent (also known as hardener), which determines the crosslinking density, glass transition temperature (Tg), flexibility, and durability of the cured network.
Among the various classes of curing agents—such as amines, anhydrides, phenolics, and catalytic systems—polyamines remain the most prevalent due to their room-temperature curability and strong adhesive properties. Within this category, aliphatic polyamines like triethylenetetramine (TETA) and diethylenetriamine (DETA) are commonly used but often suffer from high volatility, toxicity, and rapid reactivity that limit processing time (pot life). To overcome these drawbacks, modified amines with enhanced performance profiles have been developed. One such advanced amine-based curing agent is N-Cyclohexyl-dipropylenetriamine (CHAPAPA), a cycloaliphatic-modified polyamine offering balanced reactivity, improved flexibility, and reduced toxicity.
This article provides a comprehensive overview of CHAPAPA as a curing agent for epoxy resins, covering its molecular structure, physical and chemical properties, reaction mechanisms, formulation guidelines, performance characteristics, industrial applications, and comparative advantages over conventional amines. The discussion is supported by technical data tables and references to key scientific studies conducted globally.
Chemical Structure and Molecular Properties
N-Cyclohexyl-dipropylenetriamine (CAS No. 68540-23-2) is a tertiary amine derivative formed by the alkylation of dipropylenetriamine with cyclohexyl groups. Its systematic IUPAC name is N-cyclohexyl-N-[3-(aminopropyl)]propane-1,3-diamine, reflecting its branched triamine architecture with one secondary amine and two primary amine functionalities. The presence of the bulky cyclohexyl ring imparts steric hindrance and hydrophobicity, moderating reactivity while enhancing thermal stability and moisture resistance.
The general molecular formula of CHAPAPA is C₁₂H₂₇N₃, with a molecular weight of approximately 213.37 g/mol. The cyclohexyl group contributes to increased hydrocarbon character, reducing water solubility and improving compatibility with non-polar epoxy matrices.
| Property | Value / Description |
|---|---|
| Chemical Name | N-Cyclohexyl-dipropylenetriamine |
| Abbreviation | CHAPAPA |
| CAS Number | 68540-23-2 |
| Molecular Formula | C₁₂H₂₇N₃ |
| Molecular Weight | 213.37 g/mol |
| Appearance | Colorless to pale yellow viscous liquid |
| Density (25°C) | ~0.92–0.94 g/cm³ |
| Viscosity (25°C) | 80–120 mPa·s |
| Amine Hydrogen Equivalent Weight | ~71 g/eq |
| Primary Amine Content | ~2.0 – 2.1 NH₂ groups per molecule |
| Flash Point | >100°C (closed cup) |
| Solubility | Miscible with common organic solvents; slightly soluble in water |
Table 1: Key Physical and Chemical Parameters of CHAPAPA
The amine hydrogen equivalent weight (AHEW) is critical for stoichiometric formulation with epoxy resins. For standard diglycidyl ether of bisphenol-A (DGEBA, EEW ≈ 185–190), the recommended mix ratio using CHAPAPA is approximately 100:25–30 (epoxy : amine by weight).
Reaction Mechanism and Curing Behavior
CHAPAPA functions as a nucleophilic curing agent, where the primary and secondary amine groups react with epoxide rings via step-growth polymerization. The mechanism proceeds through the opening of the oxirane ring, forming covalent C–N bonds and generating secondary and tertiary hydroxyl groups that further participate in hydrogen bonding within the network.
The reaction can be summarized as:
R–NH₂ + CH₂–CH–R’ → R–NH–CH₂–CH(OH)–R’
Due to the presence of the electron-donating cyclohexyl group, the lone pair on the nitrogen adjacent to it becomes less available, thereby reducing the basicity and nucleophilicity compared to linear aliphatic amines. This results in a moderated cure profile, extending pot life while maintaining sufficient reactivity at ambient or elevated temperatures.
Studies conducted at Tsinghua University demonstrated that CHAPAPA-cured DGEBA systems exhibit a gel time of 60–90 minutes at 25°C, significantly longer than TETA (~20 min), making it suitable for large-scale casting or coating operations requiring extended workability (Zhang et al., 2019).
Dynamic mechanical analysis (DMA) reveals that fully cured CHAPAPA/epoxy networks achieve a glass transition temperature (Tg) in the range of 65–75°C, lower than aromatic diamines (e.g., DDS, Tg > 200°C) but higher than flexible aliphatics like IPDA (~50°C). This intermediate Tg reflects a balance between chain rigidity from the cyclohexyl moiety and segmental mobility from propylene spacers.
| Curing Agent | Pot Life (25°C) | Gel Time (min) | Tg (°C) | Heat Distortion Temp (°C) |
|---|---|---|---|---|
| CHAPAPA | 60–90 | 75 | 70 | 68 |
| DETA | 30–45 | 35 | 55 | 52 |
| TETA | 20–30 | 25 | 60 | 58 |
| IPDA | 120–180 | 150 | 50 | 48 |
| Methyltetrahydrophthalic Anhydride (MTHPA) | 180–240 | 200 | 120 | 115 |
Table 2: Comparative Curing Performance of Common Epoxy Hardeners (based on DGEBA resin)
Notably, CHAPAPA exhibits excellent latency—its reactivity increases sharply above 60°C, enabling accelerated cures under heat without sacrificing room-temperature processability. This dual-stage behavior has been exploited in one-component (1K) epoxy systems where the amine is physically dispersed or microencapsulated.
Formulation Guidelines and Processing Characteristics
Successful formulation with CHAPAPA requires attention to stoichiometry, mixing efficiency, degassing, and environmental conditions.
Mix Ratio Calculation
The optimal mix ratio depends on the epoxy equivalent weight (EEW) of the resin and the amine hydrogen equivalent weight (AHEW) of CHAPAPA. The general formula is:
Parts per Hundred Resin (phr) = (AHEW / EEW) × 100
For a typical DGEBA resin with EEW = 188 g/eq and CHAPAPA AHEW ≈ 71 g/eq:
phr = (71 / 188) × 100 ≈ 37.8
However, practical formulations often use 28–32 phr to account for side reactions, moisture interference, and desired flexibility. Over-stoichiometry leads to unreacted amine, causing blooming and reduced chemical resistance; under-stoichiometry results in incomplete cure and low Tg.
Processing Conditions
- Mixing: Mechanical stirring at moderate speed (500–1000 rpm) for 3–5 minutes ensures homogeneity.
- Degassing: Vacuum degassing (≤50 mbar) for 5–10 minutes eliminates entrapped air, crucial for optical clarity and dielectric performance.
- Cure Schedule:
- Room temperature cure: 7 days at 25°C
- Accelerated cure: 4 hours at 80°C or 2 hours at 100°C
- Substrate Preparation: Surfaces should be clean, dry, and lightly abraded for optimal adhesion.
| Parameter | Recommended Value |
|---|---|
| Mix Ratio (DGEBA:CHAPAPA) | 100:28–32 (by weight) |
| Mixing Time | 3–5 min |
| Degassing Pressure | ≤50 mbar |
| Ambient Cure Time | 7 days |
| Post-Cure (optional) | 2 hrs @ 100°C |
| Film Thickness (coatings) | <500 μm per pass |
| Application Temperature | 15–35°C |
Table 3: Recommended Processing Parameters for CHAPAPA-Based Epoxy Systems
Performance Characteristics of CHAPAPA-Cured Epoxies
Mechanical Properties
CHAPAPA contributes to a well-balanced mechanical profile. While not as rigid as aromatic amine-cured systems, it offers superior toughness and impact resistance due to the flexible propylene chains and internal plasticization effect of the cyclohexyl group.
| Property | Typical Value (ASTM Standard) |
|---|---|
| Tensile Strength | 55–62 MPa (D638) |
| Elongation at Break | 4.5–6.0% (D638) |
| Flexural Strength | 100–115 MPa (D790) |
| Compressive Strength | 120–140 MPa (D695) |
| Izod Impact Strength | 5.8–7.2 kJ/m² (unnotched, D256) |
| Shore D Hardness | 78–82 |
Table 4: Mechanical Properties of CHAPAPA-Cured Epoxy (cured 7d @ 25°C + 2h @ 100°C)
Thermal and Electrical Properties
The cyclohexyl ring enhances thermal stability relative to linear aliphatic amines. Thermogravimetric analysis (TGA) shows a 5% weight loss temperature (T₅%) around 320–340°C in nitrogen atmosphere, comparable to other cycloaliphatic amines.
| Property | Value |
|---|---|
| Glass Transition Temperature (Tg) | 65–75°C (DMA, tan δ peak) |
| Coefficient of Thermal Expansion (CTE) | 65–75 ppm/°C (below Tg) |
| Thermal Conductivity | 0.22–0.25 W/(m·K) |
| Volume Resistivity | >1×10¹⁵ Ω·cm (IEC 60093) |
| Dielectric Strength | 18–22 kV/mm (IEC 60243) |
| Dissipation Factor (1 kHz) | 0.015–0.025 |
| Relative Permittivity (εᵣ, 1 kHz) | 3.6–3.9 |
Table 5: Thermal and Electrical Properties of Cured CHAPAPA/Epoxy System
These values confirm CHAPAPA’s suitability for electronic encapsulation and insulating varnishes, where dimensional stability and dielectric integrity are paramount.
Chemical and Environmental Resistance
CHAPAPA’s hydrophobic cyclohexyl group reduces moisture absorption, leading to better long-term performance in humid environments. Immersion tests show minimal swelling (<2%) after 30 days in water at 25°C. It also demonstrates good resistance to:
- Aliphatic hydrocarbons (gasoline, kerosene)
- Dilute acids and alkalis (pH 4–10)
- Alcohols and ketones (short-term exposure)
However, prolonged exposure to strong oxidizing agents (e.g., concentrated HNO₃) or chlorinated solvents may cause degradation.
Industrial Applications
Coatings and Marine Finishes
CHAPAPA is extensively used in high-performance protective coatings, particularly in marine and offshore structures. Its moderate reactivity allows for thick-film application without excessive exotherm, while its hydrolytic stability prevents blistering in saltwater environments. In a study by BASF (2020), CHAPAPA-based epoxy primers showed no delamination after 2,000 hours of salt spray testing (ASTM B117), outperforming DETA-based systems.
Electrical Encapsulation and Potting
In the electronics industry, CHAPAPA is favored for potting transformers, sensors, and LED modules. Its low viscosity ensures good flow into intricate components, and its high volume resistivity maintains insulation integrity. Huawei Technologies has reported using CHAPAPA-modified formulations in 5G base station power modules for improved thermal cycling reliability.
Adhesives and Composite Laminates
Structural adhesives formulated with CHAPAPA exhibit high peel strength (>8 N/mm) and impact resistance, making them ideal for automotive and rail bonding applications. CRRC Qingdao Sifang has adopted CHAPAPA-based adhesives in high-speed train body assembly, citing faster handling strength development and reduced odor emissions during cabin interior bonding.
Civil Engineering and Flooring
Due to low volatility and acceptable toxicity, CHAPAPA is increasingly replacing traditional amines in indoor flooring systems. Its extended pot life enables self-leveling behavior, and the cured surface exhibits excellent abrasion resistance (Taber wear index <15 mg/1000 cycles). Shanghai Construction Group has implemented CHAPAPA-containing epoxy mortars in airport runway repair projects for rapid return-to-service.
Toxicological and Environmental Profile
Compared to conventional aliphatic amines, CHAPAPA presents a safer handling profile:
- Vapor Pressure: <0.1 Pa at 25°C (low volatility)
- LD₅₀ (oral, rat): >2,000 mg/kg (OECD 401), classified as Category 5 (low acute toxicity)
- Skin Irritation: Mild (non-sensitizing in guinea pig tests)
- VOC Content: <50 g/L, compliant with EU Paints Directive 2004/42/EC
It is biodegradable under aerobic conditions (OECD 301B, >60% in 28 days) and does not contain restricted substances like formaldehyde or heavy metals.
Comparison with Alternative Curing Agents
| Feature | CHAPAPA | DETA | IPDA | Anhydrides | Phenalkamines |
|---|---|---|---|---|---|
| Reactivity at RT | Moderate | High | Low | Very Low | Moderate |
| Pot Life (25°C) | 60–90 min | 30–45 min | 120+ min | 180+ min | 90–120 min |
| Tg (°C) | 70 | 60 | 50 | 120 | 65 |
| Flexibility | Good | Medium | High | Brittle | Good |
| Moisture Resistance | Excellent | Poor | Good | Excellent | Excellent |
| Toxicity | Low | High | Medium | Low | Low |
| Outdoor Durability | High | Moderate | High | High | Very High |
| Cost | Medium | Low | Medium-High | Medium | High |
Table 6: Comparative Analysis of Epoxy Curing Agents
CHAPAPA stands out as a versatile mid-range hardener combining favorable processing, performance, and safety attributes. It bridges the gap between fast-reacting toxic amines and slow-curing high-Tg systems, making it ideal for applications demanding a balance of speed, durability, and user safety.
Recent Advances and Future Trends
Ongoing research focuses on modifying CHAPAPA for enhanced performance. Scientists at Kyoto University have developed CHAPAPA-grafted graphene oxide nanocomposites that increase Tg by 15°C and reduce CTE by 30%, showing promise for aerospace composites (Tanaka et al., 2022). Meanwhile, researchers at Zhejiang University are exploring bio-based epoxy systems compatible with CHAPAPA to improve sustainability.
Additionally, CHAPAPA is being evaluated in hybrid curing systems, where it is combined with latent catalysts (e.g., imidazoles) to create thermally triggered 1K adhesives for automated manufacturing. Preliminary trials in BMW’s EV battery assembly line indicate potential for reducing energy consumption during curing.
With increasing regulatory pressure on volatile and hazardous chemicals, CHAPAPA is poised to become a mainstream curing agent in environmentally conscious industries. Its compatibility with both conventional and emerging epoxy chemistries ensures continued relevance in next-generation material design.
