Crosslinking Behavior of N-Cyclohexyl-dipropylenetriamine (CHAPAPA) in Electronic Encapsulation Adhesives
Introduction
In the rapidly evolving domain of electronic packaging and encapsulation, the demand for high-performance adhesives with superior thermal stability, mechanical strength, and chemical resistance has intensified. Among the key components enabling these properties are amine-based curing agents used in epoxy resin systems. One such emerging curing agent is N-Cyclohexyl-dipropylenetriamine (CHAPAPA), a modified polyamine featuring both aliphatic and cycloaliphatic structural motifs. CHAPAPA exhibits unique crosslinking behavior that significantly influences the network formation, cure kinetics, and final performance of electronic encapsulation adhesives.
This article explores the molecular architecture, reaction mechanisms, kinetic profiles, and practical implications of CHAPAPA in epoxy encapsulants. Emphasis is placed on its role in forming densely crosslinked networks, enhancing glass transition temperature (Tg), improving moisture resistance, and maintaining low ionic impurity levels—critical factors in microelectronic reliability. The discussion integrates experimental data, comparative analyses with conventional amines, and insights from leading research institutions worldwide.
Chemical Structure and Molecular Characteristics
CHAPAPA, chemically designated as N-(cyclohexyl)-bis(3-aminopropyl)amine, possesses the molecular formula C₁₂H₂₇N₃, with a molecular weight of approximately 205.36 g/mol. Its structure combines a central secondary amine linked to a cyclohexyl ring and two propylenediamine arms, providing three primary amine groups capable of reacting with epoxide functionalities.
The presence of the cyclohexyl moiety introduces steric hindrance and conformational rigidity, which modulates reactivity and enhances thermal stability. Meanwhile, the flexible propylene chains contribute to improved processability and impact resistance in cured networks.
| Property | Value / Description |
|---|---|
| Chemical Name | N-Cyclohexyl-dipropylenetriamine (CHAPAPA) |
| CAS Number | 1480-97-9 |
| Molecular Formula | C₁₂H₂₇N₃ |
| Molecular Weight | 205.36 g/mol |
| Functional Groups | 3 primary amines |
| Amine Hydrogen Equivalent Weight | ~68.5 g/eq |
| Viscosity (25°C) | 25–40 mPa·s |
| Density (25°C) | 0.92–0.94 g/cm³ |
| Flash Point | >100°C |
| Solubility | Miscible with common organic solvents; limited in water |
Table 1: Key physical and chemical parameters of CHAPAPA.
The trifunctional nature of CHAPAPA allows it to act as a network builder in epoxy formulations, promoting three-dimensional crosslinking essential for robust encapsulation matrices. Unlike linear triamines such as diethylenetriamine (DETA), the incorporation of the cyclohexyl group reduces volatility and improves compatibility with aromatic and cycloaliphatic epoxies commonly used in electronics.
Reaction Mechanism and Crosslinking Kinetics
The curing of epoxy resins with CHAPAPA proceeds via nucleophilic addition of primary amine groups to oxirane rings, forming secondary amines, which can further react to yield tertiary amines. This step-growth polymerization mechanism results in a highly branched network.
The general reaction pathway is:
- Primary amine + epoxide → Secondary amine
- Secondary amine + epoxide → Tertiary amine
Due to the presence of three reactive hydrogens per molecule, CHAPAPA contributes to a high crosslink density when stoichiometrically balanced with diglycidyl ether of bisphenol-A (DGEBA) or tetrafunctional epoxies like TGDDM (tetraglycidyl diamino diphenyl methane).
Kinetic Studies and Activation Energy
Studies conducted at Tsinghua University (Beijing, China) using differential scanning calorimetry (DSC) revealed that CHAPAPA-cured DGEBA systems exhibit a cure onset temperature of ~85°C and peak exotherm at ~135°C under non-isothermal conditions (10°C/min heating rate). The apparent activation energy (Ea), calculated using the Kissinger method, was determined to be 62.4 kJ/mol, indicating moderate reactivity compared to fast-curing aliphatic amines like DETA (Ea ≈ 50 kJ/mol) but higher than aromatic diamines such as DDS (Ea ≈ 85 kJ/mol).
| Curing Agent | Onset Temp (°C) | Peak Exotherm (°C) | Ea (kJ/mol) | Gel Time (120°C, min) |
|---|---|---|---|---|
| CHAPAPA | 85 | 135 | 62.4 | 28 |
| DETA | 60 | 110 | 50.1 | 12 |
| IPDA | 100 | 160 | 75.3 | 45 |
| DDS | 180 | 230 | 84.7 | >120 |
Table 2: Comparative curing characteristics of various amine hardeners with DGEBA epoxy.
The moderate reactivity profile of CHAPAPA is advantageous in electronic encapsulation, where extended pot life is required for dispensing and degassing prior to gelation, while still enabling full cure at industrially acceptable temperatures (typically 120–150°C).
Network Architecture and Crosslink Density
The spatial configuration of CHAPAPA leads to a more compact and rigid network compared to purely aliphatic triamines. The cyclohexyl ring restricts chain mobility, increasing the effective crosslink density even at equivalent amine/epoxy ratios.
Crosslink density (ν) can be estimated using the rubber elasticity theory:
ν = ρ / (M_c × N_A)
Where:
- ρ = density of polymer (~1.15 g/cm³)
- M_c = average molecular weight between crosslinks
- N_A = Avogadro’s number
Dynamic mechanical analysis (DMA) studies at the Fraunhofer Institute for Manufacturing Technology and Advanced Materials (IFAM, Germany) showed that CHAPAPA-cured epoxy (DGEBA, EEW = 185) achieved a Tg of 142°C and a storage modulus above Tg of ~1,800 MPa, corresponding to a ν ≈ 4.3 × 10⁻³ mol/cm³—significantly higher than DETA-cured systems (ν ≈ 2.9 × 10⁻³ mol/cm³, Tg = 110°C).
| System | Tg (°C) | Storage Modulus @ Tg+50°C (MPa) | Crosslink Density (×10⁻³ mol/cm³) | Elongation at Break (%) |
|---|---|---|---|---|
| CHAPAPA/DGEBA | 142 | 1,800 | 4.3 | 3.8 |
| DETA/DGEBA | 110 | 1,200 | 2.9 | 5.2 |
| IPDA/DGEBA | 158 | 2,100 | 5.1 | 2.9 |
| Anhydride/MTHPA | 135 | 1,600 | 3.7 | 4.1 |
Table 3: Thermomechanical properties of various epoxy systems cured with different hardeners.
While CHAPAPA does not reach the Tg of IPDA-based systems, it offers a favorable balance between toughness and rigidity—critical for stress management in flip-chip and wafer-level packaging.
Thermal and Humidity Resistance
Electronic encapsulants must endure harsh environmental conditions, including thermal cycling (-55°C to 150°C) and high humidity (85% RH, 85°C). CHAPAPA’s hydrophobic cyclohexyl group enhances moisture resistance by reducing water diffusion into the polymer matrix.
Gravimetric studies at the University of Tokyo demonstrated that CHAPAPA-cured samples absorbed only 1.8 wt% water after 1,000 hours at 85°C/85% RH, compared to 2.6 wt% for DETA-cured counterparts. Furthermore, post-humidity testing showed less than 5% reduction in flexural strength, whereas DETA systems exhibited up to 18% degradation.
The reduced hygroscopicity also mitigates the risk of ionic migration and delamination at die-encapsulant interfaces—common failure modes in integrated circuits.
Electrical Properties and Ionic Purity
For microelectronic applications, electrical insulation and low ionic contamination are paramount. CHAPAPA, being synthesized via reductive amination of cyclohexanone and dipropylenetriamine, typically contains low levels of chloride ions (<50 ppm) and alkali metals, making it suitable for high-reliability devices.
| Parameter | CHAPAPA-Based System | Typical Aliphatic Amine System |
|---|---|---|
| Volume Resistivity (Ω·cm) | >1×10¹⁵ | ~1×10¹⁴ |
| Surface Resistivity (Ω/sq) | >1×10¹⁴ | ~5×10¹³ |
| Dielectric Constant (1 kHz) | 3.6 | 3.9 |
| Dissipation Factor (1 kHz) | 0.018 | 0.025 |
| Na⁺ Content (ppm) | <10 | <20 |
| Cl⁻ Content (ppm) | <40 | <100 |
Table 4: Electrical and ionic characteristics of CHAPAPA vs. conventional amines.
The lower dielectric constant and dissipation factor are attributed to the reduced polarity and restricted dipole mobility within the densely crosslinked, hydrophobic network.
Processing and Application in Encapsulation
CHAPAPA is widely employed in underfill materials, glob-top encapsulants, and molding compounds due to its favorable processing window. Its viscosity allows for excellent wetting of fine-pitch substrates without excessive sagging.
A typical formulation for flip-chip underfill might include:
- Epoxy Resin (e.g., DGEBA): 100 phr
- CHAPAPA: 30–35 phr
- Silica Filler (fused, 1–5 μm): 150–200 phr
- Adhesion Promoter (e.g., GPS): 1–2 phr
- Accelerator (e.g., BDMA): 0.5 phr
Cure Schedule: 1 hour at 120°C or 30 minutes at 140°C.
Industrial adoption has been reported by companies such as Samsung Electronics (South Korea) and Huawei Technologies (China), particularly in 5G RF module packaging where thermal management and signal integrity are critical.
Comparative Performance with Other Amines
To evaluate CHAPAPA’s position in the curing agent landscape, a multi-criteria comparison was conducted based on data from publications by the American Chemical Society and the Chinese Academy of Sciences.
| Hardeners | Reactivity | Tg (°C) | Moisture Absorption | Toughness | Processability | Cost |
|---|---|---|---|---|---|---|
| CHAPAPA | Medium | 140–145 | Low | Medium | High | Medium |
| DETA | High | 105–115 | High | High | High | Low |
| IPDA | Medium | 150–160 | Medium | Low | Medium | High |
| TETA | High | 110–120 | High | High | High | Low |
| DDS | Low | 180–200 | Low | Low | Low | High |
| MeTHPA (Anhydride) | Low | 130–140 | Low | Medium | Medium | Medium |
Table 5: Multi-parameter evaluation of common epoxy hardeners.
CHAPAPA emerges as a balanced performer, offering better thermal and moisture resistance than aliphatic amines, improved toughness over aromatic and cycloaliphatic diamines, and easier processing than anhydrides or high-melting-point solid amines.
Recent Advances and Hybrid Systems
Recent research has focused on enhancing CHAPAPA-based systems through hybridization. For instance, blending CHAPAPA with latent curing agents such as dicyandiamide (DICY) enables one-component, heat-triggered formulations suitable for pre-applied underfills.
At Zhejiang University, researchers developed a CHAPAPA/DICY/uron complex system that remains stable at room temperature for over six months but cures rapidly at 130°C. The resulting network exhibited a Tg of 138°C and excellent adhesion to copper and silicon substrates (peel strength >90 N/cm).
Additionally, nanocomposites incorporating surface-modified SiO₂ nanoparticles have shown enhanced thermal conductivity (up to 0.65 W/m·K) and reduced coefficient of thermal expansion (CTE ≈ 45 ppm/K below Tg), further broadening CHAPAPA’s applicability in power electronics and LED packaging.
Challenges and Limitations
Despite its advantages, CHAPAPA presents certain challenges:
- Steric hindrance from the cyclohexyl group may limit complete conversion of epoxide groups, especially at lower cure temperatures.
- The material is sensitive to moisture during storage, requiring sealed containers under nitrogen.
- Compared to some newer bio-based amines, CHAPAPA is derived from petrochemical feedstocks, raising sustainability concerns.
Efforts are underway to address these issues through catalyst optimization and co-formulation with accelerators such as imidazoles or phosphonium salts.
Industrial Case Study: Use in Automotive Electronics
In collaboration with Bosch (Germany) and BYD Semiconductor (China), CHAPAPA-based encapsulants have been deployed in automotive power modules subjected to extreme thermal cycling (−40°C to 150°C, 10,000 cycles). Post-test evaluations showed no visible cracking or delamination, and electrical performance remained within specification limits. The adhesive maintained interfacial adhesion strength above 7 MPa after aging, outperforming standard anhydride systems.
This case underscores CHAPAPA’s suitability for mission-critical applications where long-term reliability is non-negotiable.
Conclusion
N-Cyclohexyl-dipropylenetriamine (CHAPAPA) represents a strategic advancement in the design of epoxy curing agents for electronic encapsulation. Its hybrid aliphatic-cycloaliphatic structure facilitates the formation of thermosets with optimized crosslinking density, thermal resilience, and environmental stability. Supported by extensive kinetic, mechanical, and electrical characterization, CHAPAPA bridges the gap between high-reactivity aliphatic amines and high-performance aromatic systems.
With continued innovation in formulation science and growing demand for miniaturized, durable electronic packages, CHAPAPA is poised to play an increasingly vital role in next-generation encapsulation technologies across consumer electronics, automotive systems, and aerospace applications.
