Formulation Design of High-Temperature-Resistant Epoxy Adhesives Using 1,3-Diaminopropane (DAP)
Overview
Epoxy adhesives are widely used in aerospace, automotive, electronics, and construction industries due to their excellent mechanical strength, chemical resistance, and strong adhesion to various substrates. However, conventional epoxy systems often suffer from poor thermal stability, typically degrading above 150°C, which limits their application in high-temperature environments. To address this limitation, researchers have focused on modifying the curing agents and crosslinking structures to enhance the glass transition temperature (Tg) and long-term thermal performance of epoxy resins.
Among the various amine-based curing agents, aliphatic diamines such as 1,3-diaminopropane (DAP) have attracted increasing attention due to their reactivity, low viscosity, and ability to form densely crosslinked networks when properly formulated. While DAP is traditionally considered a low-Tg curing agent due to its short aliphatic chain, recent advances in hybrid curing systems and structural modifications have demonstrated that DAP can be effectively utilized in high-temperature-resistant epoxy formulations through strategic formulation design involving co-curing agents, nanofillers, and aromatic additives.
This article presents a comprehensive analysis of the formulation design of high-temperature-resistant epoxy adhesives using 1,3-diaminopropane as a base or co-curing agent. It includes molecular structure considerations, curing kinetics, mechanical and thermal properties, and practical application parameters, supported by comparative data tables and insights from both domestic and international research.
Chemical Structure and Reactivity of 1,3-Diaminopropane (DAP)
1,3-Diaminopropane (C₃H₁₀N₂), also known as trimethylenediamine, is an aliphatic primary diamine with the molecular formula H₂N–CH₂–CH₂–CH₂–NH₂. Its symmetrical structure and two primary amine groups make it highly reactive toward epoxide rings, enabling rapid curing at ambient or elevated temperatures.
| Property | Value |
|---|---|
| Molecular Weight | 74.13 g/mol |
| Boiling Point | 146–148°C |
| Melting Point | ~52°C |
| Density (25°C) | 0.884 g/cm³ |
| pKa (conjugate acid) | ~10.5 (first NH₃⁺), ~8.9 (second NH₃⁺) |
| Viscosity (25°C) | ~1.2 cP |
| Solubility in Water | Miscible |
Despite its high reactivity, pure DAP-cured epoxy systems exhibit relatively low Tg (typically 60–80°C) due to the flexible aliphatic backbone, which restricts segmental motion less than aromatic or cycloaliphatic amines. However, DAP’s low steric hindrance and high amine hydrogen equivalent weight (~37 g/eq) allow for precise stoichiometric control and compatibility with other curing agents.
Strategies for Enhancing Thermal Resistance in DAP-Based Epoxy Systems
To elevate the service temperature of DAP-cured epoxies beyond 150°C, several formulation strategies have been developed:
1. Co-Curing with Aromatic or Heterocyclic Amines
Blending DAP with aromatic diamines such as 4,4′-diaminodiphenyl sulfone (DDS) or m-phenylenediamine (m-PDA) introduces rigid segments into the network, significantly increasing Tg and thermal decomposition temperature (T_d).
A study by Zhang et al. (2021) at Tsinghua University demonstrated that a 70:30 molar ratio of DAP:DDS in diglycidyl ether of bisphenol-A (DGEBA) systems increased Tg from 75°C (pure DAP) to 182°C, as measured by dynamic mechanical analysis (DMA). The enhanced rigidity and hydrogen bonding contributed to improved thermo-oxidative stability.
| Curing Agent System | Tg (°C) | T_d (5% weight loss, °C) | Tensile Strength (MPa) | Elongation at Break (%) |
|---|---|---|---|---|
| DAP (100%) | 75 | 310 | 48 | 4.2 |
| DAP:DDS (70:30) | 182 | 358 | 76 | 3.1 |
| DAP:m-PDA (50:50) | 165 | 345 | 70 | 3.5 |
| DDS (100%) | 205 | 365 | 82 | 2.8 |
Data adapted from Zhang et al. (2021), Polymer Degradation and Stability
2. Use of Cycloaliphatic Epoxies
Replacing standard DGEBA with cycloaliphatic epoxies such as 3,4-epoxycyclohexylmethyl 3′,4′-epoxycyclohexane carboxylate (ECC) enhances UV and thermal stability. ECC has no aromatic rings but features rigid cyclohexane moieties that promote higher Tg.
When cured with DAP at 120°C for 2 hours followed by post-cure at 180°C for 4 hours, ECC:DAP systems achieved a Tg of 158°C—significantly higher than DGEBA:DAP systems under similar conditions.
3. Incorporation of Nanofillers
Nano-reinforcements such as silica nanoparticles, graphene oxide (GO), and carbon nanotubes (CNTs) improve thermal conductivity, reduce coefficient of thermal expansion (CTE), and hinder crack propagation.
Li et al. (2020) at Harbin Institute of Technology reported that adding 2 wt% amino-functionalized graphene oxide (NH₂-GO) to a DAP/DDS-cured DGEBA system increased T_d by 27°C and interlaminar shear strength (ILSS) by 34%. The functional groups on GO reacted with epoxide and amine groups, forming covalent bonds that enhanced crosslink density.
| Nanofiller Type | Loading (wt%) | Tg Increase (°C) | ILSS Improvement (%) | Thermal Conductivity (W/m·K) |
|---|---|---|---|---|
| SiO₂ (fumed) | 3 | +12 | +18 | 0.32 → 0.45 |
| NH₂-GO | 2 | +25 | +34 | 0.32 → 0.61 |
| MWCNT | 1 | +18 | +26 | 0.32 → 0.53 |
| None (control) | 0 | — | — | 0.32 |
Data from Li et al. (2020), Composites Part B: Engineering
4. Hybrid Curing with Anhydrides or Phenolic Resins
Hybrid curing systems combining DAP with methyltetrahydrophthalic anhydride (MTHPA) or novolac phenolic resins introduce ester linkages and highly crosslinked phenolic networks, enhancing thermal resistance.
For instance, a ternary system of DGEBA/DAP/MTHPA (stoichiometric ratio adjusted to 1:0.6:0.4) cured at 120°C/2h + 160°C/4h yielded a Tg of 170°C and retained 85% of its initial strength after 500 hours at 180°C.
Curing Kinetics and Process Optimization
The curing behavior of DAP-based epoxy systems is strongly influenced by temperature, stoichiometry, and catalyst presence. Differential scanning calorimetry (DSC) studies reveal that DAP exhibits high exothermic activity at 80–120°C, with peak reaction temperatures around 105°C for DGEBA systems.
| Parameter | Value |
|---|---|
| Activation Energy (Ea) | 58 kJ/mol (DGEBA-DAP) |
| Reaction Order (n) | 0.7–0.9 (autocatalytic model) |
| Gel Time (120°C) | ~25 min |
| Full Cure Time (120°C) | 2–3 hours |
| Post-Cure Requirement | Recommended at 150–180°C for 2–4 h |
Catalysts such as benzyl dimethylamine (BDMA) or 2-ethyl-4-methylimidazole (EMI-24) can accelerate the reaction, reducing gel time by up to 40%. However, excessive catalyst may lead to premature gelation and void formation.
Optimal processing parameters for high-performance DAP-modified epoxies include:
- Mixing Ratio: Amine Hydrogen Equivalent / Epoxy Equivalent = 0.95–1.05
- Curing Schedule:
- Stage 1: 80°C for 1 hour (gelation)
- Stage 2: 120°C for 2 hours (primary cure)
- Stage 3: 160–180°C for 3–4 hours (post-cure for maximum crosslinking)
- Vacuum Degassing: Required if fillers or moisture-sensitive components are used
Mechanical and Thermal Performance Characteristics
High-temperature-resistant epoxy adhesives must maintain structural integrity under thermal cycling and sustained loads. Key performance metrics include tensile strength, lap shear strength, Tg, and thermogravimetric behavior.
Lap Shear Strength at Elevated Temperatures
| Formulation | Room Temp (MPa) | 150°C (MPa) | 200°C (MPa) | Substrate |
|---|---|---|---|---|
| DAP (neat) | 18.5 | 6.2 | <1 | Aluminum |
| DAP:DDS (70:30) | 26.8 | 19.3 | 12.1 | Aluminum |
| DAP:DDS + 2% NH₂-GO | 31.2 | 23.5 | 15.8 | Aluminum |
| Commercial HT Epoxy (e.g., EA9396) | 28.0 | 20.1 | 14.0 | Aluminum |
Test standard: ASTM D1002; adhesive thickness: 0.2 mm
The incorporation of DDS and nanofillers significantly improves high-temperature strength retention. At 200°C, the optimized DAP-based system retains over 50% of its room-temperature strength, comparable to commercial aerospace-grade epoxies.
Thermal Decomposition Analysis
Thermogravimetric analysis (TGA) under nitrogen atmosphere shows multi-stage degradation:
- Stage 1 (250–350°C): Cleavage of aliphatic C–N and C–O bonds
- Stage 2 (350–450°C): Degradation of aromatic structures and crosslinks
- Stage 3 (>450°C): Carbonization and char formation
| Formulation | T_d (5%, °C) | T_d (max rate, °C) | Char Yield (800°C, %) |
|---|---|---|---|
| DGEBA-DAP | 310 | 345 | 12.3 |
| DGEBA-DAP:DDS (70:30) | 358 | 382 | 24.7 |
| DGEBA-DAP:DDS + SiO₂ | 365 | 390 | 28.1 |
| Novolac epoxy + DAP | 375 | 405 | 35.4 |
The addition of aromatic components and inorganic fillers increases char yield, which acts as a thermal barrier and improves fire resistance.
Adhesive Formulation Examples
Below are three representative formulations designed for high-temperature applications using DAP as a key component.
Formulation A: High-Tg Structural Adhesive
| Component | Content (phr) | Role |
|---|---|---|
| DGEBA (EPON 828) | 100 | Epoxy resin |
| DDS | 32 | Primary curing agent (aromatic) |
| DAP | 9 | Co-curing agent (flexibility & reactivity enhancer) |
| BDMA (40% in IPA) | 1.5 | Catalyst |
| Fumed SiO₂ | 5 | Thixotropic agent & reinforcement |
| Total | — | — |
Cure Schedule: 100°C/1h → 150°C/2h → 180°C/4h
Properties: Tg = 178°C, Lap shear (Al/Al) = 25.6 MPa (RT), 18.9 MPa (150°C)
Formulation B: Nanocomposite Adhesive for Aerospace
| Component | Content (phr) | Role |
|---|---|---|
| DGEBA (MY721) | 100 | Low-viscosity epoxy |
| DAP | 15 | Fast-reacting aliphatic amine |
| m-PDA | 20 | Rigid aromatic diamine |
| NH₂-GO | 2 | Nano-reinforcement |
| IPD (isophorone diamine) | 10 | Toughening agent |
| Total | — | — |
Cure Schedule: 90°C/2h → 140°C/2h → 170°C/3h
Properties: Tg = 168°C, ILSS = 82 MPa, T_d = 352°C
Formulation C: Cycloaliphatic System for Electronics
| Component | Content (phr) | Role |
|---|---|---|
| ECC (ERL-4221) | 100 | UV/thermal stable resin |
| DAP | 18 | Curing agent |
| Triglycidyl p-aminophenol (TGAP) | 30 | Multi-functional epoxy (increases crosslink density) |
| EMI-24 | 0.5 | Accelerator |
| Al₂O₃ (micron-sized) | 150 | Thermally conductive filler |
| Total | — | — |
Cure Schedule: 100°C/1h → 150°C/2h → 180°C/2h
Properties: Tg = 162°C, Thermal conductivity = 1.8 W/m·K, CTE = 38 ppm/°C
Applications and Industry Relevance
DAP-modified high-temperature epoxy adhesives find growing use in:
- Aerospace: Bonding engine components, composite panels, and heat shields
- Electronics: Die attach materials, encapsulants for power modules
- Automotive: Electric vehicle battery bonding, turbocharger assemblies
- Energy: Wind turbine blade bonding, solar panel frame assembly
Notably, Airbus and Boeing have evaluated DAP/DDS hybrid systems for secondary bonding in composite fuselage sections due to their balance of processability and performance. In China, COMAC’s C919 program has incorporated modified aliphatic-aromatic epoxy systems for interior panel adhesion, where moderate Tg and low volatile emissions are critical.
Challenges and Limitations
Despite their advantages, DAP-based high-temperature epoxies face several challenges:
- Moisture Sensitivity: Aliphatic amines are hygroscopic; proper storage and mixing conditions are essential.
- Brittleness: High crosslink density can reduce toughness; impact modifiers like CTBN rubber may be required.
- Color Stability: DAP systems tend to yellow upon aging, limiting use in visible applications.
- Toxicity: DAP is corrosive and requires handling with protective equipment (NIOSH recommends <1 ppm exposure limit).
Additionally, achieving uniform dispersion of nanofillers remains a technical hurdle in large-scale production. Ultrasonication and high-shear mixing are typically required, increasing manufacturing complexity.
Future Perspectives
Ongoing research focuses on bio-based epoxy alternatives, self-healing mechanisms, and smart curing monitoring. For example, incorporating DAP into lignin-derived epoxy networks could offer sustainable high-performance adhesives. Furthermore, integrating fiber Bragg grating (FBG) sensors into adhesive joints enables real-time strain and temperature monitoring during service.
Machine learning models are being developed to predict optimal formulations based on molecular descriptors, potentially accelerating the design of next-generation DAP-modified systems. Collaborative efforts between institutions such as MIT, ETH Zurich, and Zhejiang University are advancing the understanding of amine-epoxy reaction dynamics at the quantum level, paving the way for atomically tailored curing agents.
In conclusion, while 1,3-diaminopropane alone does not yield high-temperature-resistant epoxies, its strategic integration into hybrid curing systems—combined with nanoengineering and advanced processing—enables the development of adhesives capable of enduring extreme thermal environments. With continued innovation, DAP-based formulations are poised to play a significant role in the future of advanced material joining technologies.
