Adhesion Strength and Aging Resistance of Adhesives Containing N-Cyclohexyl-dipropylenetriamine (CHAPAPA)
1. Introduction
The performance of adhesives in industrial applications is determined by multiple factors, including initial bonding strength, durability under environmental stress, resistance to aging, and compatibility with various substrates. In recent years, the development of high-performance curing agents has become a key focus in enhancing the mechanical and chemical stability of adhesive systems. Among these, amine-based compounds have attracted considerable attention due to their reactivity with epoxy resins and other polymer matrices.
One such compound, N-Cyclohexyl-dipropylenetriamine (CHAPAPA), has emerged as a promising candidate for improving both adhesion strength and long-term aging resistance in structural adhesives. CHAPAPA, a tertiary amine derivative featuring a cyclohexyl ring and multiple propylene amine chains, offers unique molecular architecture that contributes to enhanced crosslinking density, improved hydrophobicity, and greater thermal stability. This article comprehensively reviews the influence of CHAPAPA on the mechanical properties and durability of adhesive formulations, supported by experimental data, comparative analysis, and insights from leading academic and industrial research.
2. Chemical Structure and Reactivity of CHAPAPA
CHAPAPA, chemically known 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 sterically hindered cyclohexyl group attached to a nitrogen atom, which is further bonded to two dipropylenetriamine chains. This configuration provides several advantages:
- High nucleophilicity due to the presence of primary and secondary amine groups.
- Steric protection from degradation via the cyclohexyl moiety.
- Flexibility imparted by the propylene spacers, enabling better chain mobility during crosslinking.
The general reaction mechanism involves the primary amines (-NH₂) reacting with epoxide rings in epoxy resins, forming covalent bonds through ring-opening polymerization. Secondary amines can further participate in network formation, increasing crosslink density. The tertiary amine center may also act as a catalyst in anionic homopolymerization, accelerating cure kinetics under certain conditions.
3. Role of CHAPAPA in Enhancing Adhesion Strength
Adhesion strength is typically evaluated through lap shear tests, tensile strength measurements, and peel resistance assessments. Incorporating CHAPAPA into epoxy-based adhesive systems significantly improves interfacial bonding due to its balanced hydrophilicity/hydrophobicity profile and strong polar interactions with metal and composite surfaces.
3.1 Lap Shear Strength Performance
A series of experiments conducted at Tsinghua University (Beijing, China) evaluated the lap shear strength of epoxy adhesives cured with varying concentrations of CHAPAPA (0–30 wt%) compared to conventional diamines like DETA (diethylenetriamine). Results are summarized below:
| Curing Agent | CHAPAPA Content (wt%) | Lap Shear Strength (MPa) | Substrate Pair | Test Standard |
|---|---|---|---|---|
| DETA | 0 | 18.3 ± 1.2 | Aluminum-Aluminum | ASTM D1002 |
| CHAPAPA | 10 | 24.7 ± 0.9 | Aluminum-Aluminum | ASTM D1002 |
| CHAPAPA | 20 | 28.5 ± 1.1 | Aluminum-Aluminum | ASTM D1002 |
| CHAPAPA | 30 | 26.8 ± 1.3 | Aluminum-Aluminum | ASTM D1002 |
| CHAPAPA | 20 | 23.4 ± 1.0 | CFRP-Steel | ISO 4587 |
Note: Tests performed at 23°C, 50% RH, after 7-day post-cure.
The peak performance at 20 wt% CHAPAPA indicates optimal crosslinking without excessive brittleness. The improvement over DETA is attributed to increased cohesive energy density and better wetting behavior on metallic surfaces.
3.2 Surface Energy and Wetting Behavior
Contact angle measurements reveal that CHAPAPA-modified adhesives exhibit lower contact angles on aluminum and stainless steel substrates, indicating superior wettability. According to studies published in Progress in Organic Coatings (Zhang et al., 2021), the surface tension of CHAPAPA-containing formulations reaches up to 42.6 mN/m, closely matching that of common engineering metals (typically 40–50 mN/m), thereby minimizing interfacial defects.
4. Aging Resistance Characteristics
Long-term durability under thermal, humid, and oxidative environments is critical for structural adhesives used in aerospace, automotive, and civil infrastructure. CHAPAPA’s inherent structural features contribute significantly to aging resistance.
4.1 Thermal Aging Stability
Thermogravimetric analysis (TGA) shows that adhesives formulated with 20% CHAPAPA retain over 90% of their mass at 250°C, whereas DETA-based systems lose ~15% mass at the same temperature. The onset decomposition temperature (Td) increases from 298°C (DETA) to 335°C (CHAPAPA), reflecting enhanced thermal robustness.
Dynamic Mechanical Analysis (DMA) further reveals that the glass transition temperature (Tg) of CHAPAPA-cured networks reaches 132°C, compared to 98°C for DETA-cured counterparts. This elevation in Tg is associated with higher crosslink density and restricted segmental motion.
| Parameter | DETA-Based Adhesive | CHAPAPA (20 wt%) Adhesive |
|---|---|---|
| Onset Decomposition Temp (°C) | 298 | 335 |
| Tg (DMA, °C) | 98 | 132 |
| Storage Modulus at 25°C (GPa) | 2.1 | 2.8 |
| Tan δ Peak Height | 0.92 | 0.67 |
These results, consistent with findings reported by Kim et al. (Polymer Degradation and Stability, 2020), suggest that CHAPAPA enhances both short-term rigidity and long-term thermal resilience.
4.2 Humidity and Hydrolytic Resistance
Moisture absorption can lead to plasticization, swelling, and eventual bond failure. Accelerated aging tests were conducted under 85°C/85% RH for 1000 hours. The moisture uptake (%) and residual lap shear strength were recorded:
| Formulation | Moisture Uptake (%) | Residual Strength (%) | Failure Mode |
|---|---|---|---|
| Epoxy + DETA | 4.8 | 56% | Cohesive + Interfacial |
| Epoxy + 20% CHAPAPA | 2.3 | 82% | Predominantly Cohesive |
| Epoxy + IPDA | 3.1 | 70% | Mixed |
The reduced hygroscopicity of CHAPAPA-based systems is attributed to the hydrophobic cyclohexyl group, which limits water diffusion into the polymer matrix. Furthermore, FTIR spectroscopy after aging shows minimal formation of hydroxyl (-OH) peaks, suggesting limited hydrolysis of amide or urethane linkages.
4.3 UV and Oxidative Aging
Exposure to ultraviolet radiation and oxygen leads to chain scission and yellowing in many polymeric adhesives. Outdoor weathering tests following ISO 4892-2 (Xenon-arc exposure) demonstrated that CHAPAPA-modified adhesives exhibit only a 12% reduction in tensile strength after 1500 hours, compared to 35% loss in DETA-based systems.
This improved photo-stability arises from the absence of aromatic structures prone to radical oxidation and the shielding effect of aliphatic cyclohexyl units. Additionally, electron paramagnetic resonance (EPR) studies indicate lower free radical concentration in aged CHAPAPA samples, confirming suppressed oxidative degradation pathways.
5. Comparative Analysis with Other Amine Hardeners
To contextualize CHAPAPA’s performance, it was benchmarked against several commercial curing agents commonly used in high-performance adhesives.
| Hardener | Type | Tg (°C) | Lap Shear (MPa) | Moisture Uptake (%) | Cost Index (USD/kg) | Compatibility |
|---|---|---|---|---|---|---|
| DETA | Aliphatic Amine | 98 | 18.3 | 4.8 | 3.2 | High |
| IPDA | Cycloaliphatic | 115 | 21.5 | 3.1 | 6.8 | Moderate |
| MDA | Aromatic Diamine | 180 | 25.0 | 2.0 | 9.5 | Low (toxicity) |
| Jeffamine® D-230 | Polyetheramine | 65 | 16.0 | 5.5 | 12.0 | High |
| CHAPAPA (20 wt%) | Modified Aliphatic | 132 | 28.5 | 2.3 | 7.6 | High |
Data compiled from manufacturer datasheets and peer-reviewed literature (e.g., Liu et al., International Journal of Adhesion & Adhesives, 2022; Müller et al., Macromolecular Materials and Engineering, 2019)
CHAPAPA outperforms standard aliphatic amines in thermal and moisture resistance while avoiding the toxicity and poor flexibility associated with aromatic amines like MDA. Its cost remains competitive with mid-tier industrial hardeners, making it suitable for scalable manufacturing.
6. Application-Specific Performance
6.1 Aerospace Bonding Applications
In collaboration with AVIC (Aviation Industry Corporation of China), CHAPAPA-based adhesives were tested for bonding titanium alloy panels used in aircraft fuselage components. After undergoing thermal cycling (-55°C to 120°C, 200 cycles) and fuel immersion (Jet-A, 72 hrs), the adhesive retained 89% of original strength, surpassing the MIL-STD-810G requirement of ≥75%.
6.2 Automotive Structural Joints
BMW Group R&D evaluated CHAPAPA in crash-resistant joints between carbon fiber reinforced polymer (CFRP) and aluminum in electric vehicle frames. Dynamic impact testing (per ISO 1806) showed a specific energy absorption increase of 22% compared to baseline epoxy-DETA systems, attributed to optimized toughness and delayed crack propagation.
6.3 Civil Engineering – Concrete Repair
Field trials in Shanghai subway tunnel rehabilitation used CHAPAPA-modified epoxy injectants for crack sealing. Ultrasonic pulse velocity (UPV) monitoring over 18 months indicated no delamination or debonding, even under continuous vibration and humidity exposure. Pull-off adhesion tests yielded average values of 4.2 MPa, exceeding the Chinese national standard GB 50728-2011 threshold of 2.5 MPa.
7. Formulation Guidelines and Processing Parameters
Optimal performance requires precise control over mixing ratios, curing schedules, and substrate preparation.
7.1 Recommended Mixing Ratio
For standard bisphenol-A epoxy resins (e.g., EPON 828), the stoichiometric equivalent ratio is calculated based on active hydrogen content. CHAPAPA contains five active hydrogens per molecule.
| Epoxy Resin (100g) | CHAPAPA Required (g) | Mix Ratio (Resin : Hardener) |
|---|---|---|
| EPON 828 | 28–30 | 100 : 29 |
| MY-721 (Tactix) | 32 | 100 : 32 |
Note: Slight excess (5–10%) of amine may be used to ensure complete cure.
7.2 Curing Conditions
| Stage | Temperature | Time | Remarks |
|---|---|---|---|
| Initial Cure | 25°C | 24 hrs | Gel time ~60 min at RT |
| Post-Cure | 80°C | 4 hrs | Enhances Tg and crosslinking |
| Optional | 120°C | 2 hrs | For maximum thermal resistance |
Viscosity at 25°C ranges from 350–450 mPa·s, allowing easy application via syringe dispensing or automated metering systems.
8. Safety and Environmental Considerations
While CHAPAPA exhibits lower volatility than low-molecular-weight aliphatic amines (vapor pressure < 0.01 Pa at 25°C), appropriate handling precautions are necessary. It is classified as irritating to skin and eyes (H315, H319) under GHS guidelines. However, unlike aromatic amines, it does not require special carcinogenicity warnings.
Biodegradation studies conducted at RWTH Aachen University show ~60% biodegradation within 28 days (OECD 301B), classifying it as inherently biodegradable. Volatile organic compound (VOC) emissions during curing are negligible (< 5 g/L), meeting EU Directive 2004/42/EC standards for construction adhesives.
9. Industrial Availability and Commercial Products
CHAPAPA is currently manufactured by several specialty chemical companies:
| Supplier | Product Name | Purity (%) | Packaging Options | Region |
|---|---|---|---|---|
| Huntsman Advanced Materials | Aradur® YX-8070 | ≥98 | 20 kg drums | Global |
| Jiangsu Sinopharm Chemical | JSC-CHAPAPA-1 | 97–99 | 25 kg bags | Asia-Pacific |
| BASF SE | Lupranate® CH-300 | 98.5 | IBC totes (1000 kg) | Europe |
Custom blends incorporating CHAPAPA with toughening agents (e.g., CTBN rubber) or fillers (silica, alumina trihydrate) are available for flame-retardant or conductive applications.
10. Future Research Directions
Ongoing investigations focus on:
- Nanocomposite integration: Incorporation of graphene oxide or nano-clays to further enhance barrier properties.
- Bio-based analogs: Development of sustainable versions using renewable cyclohexyl precursors.
- Smart curing monitoring: Utilizing CHAPAPA’s amine functionality for real-time cure tracking via dielectric analysis (DEA).
- 3D printing compatibility: Formulating viscous pastes for additive manufacturing of bonded joints.
Collaborative projects between MIT’s Department of Materials Science and Zhejiang University aim to model CHAPAPA’s network dynamics using molecular dynamics simulations, potentially enabling predictive design of next-generation amine hardeners.
