Rheological and Curing Characteristics of Waterborne Epoxy Systems Modified with N-Cyclohexyl-dipropylenetriamine (CHAPAPA)
1. Introduction
Waterborne epoxy resins have gained increasing attention in the field of polymer science and industrial coatings due to their environmental friendliness, low volatile organic compound (VOC) emissions, and excellent mechanical and chemical resistance properties. Compared to traditional solvent-based epoxy systems, waterborne variants offer a sustainable alternative without compromising performance. However, challenges remain in balancing viscosity, curing behavior, and film formation, especially when modifying amine hardeners to achieve optimal crosslinking efficiency.
One promising approach involves the use of functionalized amine curing agents such as N-Cyclohexyl-dipropylenetriamine (CHAPAPA), a tertiary amine-containing polyamine that combines hydrophobic cyclohexyl groups with flexible propylene chains. This structure enables improved compatibility with waterborne epoxy dispersions while influencing both rheological flow and curing kinetics. This article presents a comprehensive analysis of the rheological and curing characteristics of waterborne epoxy systems modified with CHAPAPA, supported by experimental data, comparative studies, and theoretical models drawn from international research.
2. Chemical Structure and Properties of CHAPAPA
N-Cyclohexyl-dipropylenetriamine (CHAPAPA) is a branched aliphatic triamine with the molecular formula C₁₁H₂₇N₃. Its systematic IUPAC name is N¹-(cyclohexylmethyl)-N¹,N³-bis(3-aminopropyl)propane-1,3-diamine, though it is commonly abbreviated based on its structural motifs. The molecule features:
- A central cyclohexyl ring providing steric bulk and hydrophobic character,
- Two dipropylenetriamine arms offering multiple secondary and primary amine groups for epoxy ring opening,
- Tertiary nitrogen facilitating catalytic activity during curing.
The presence of both hydrophilic (amine) and hydrophobic (cyclohexyl) moieties allows CHAPAPA to act as an emulsifier and reactive hardener simultaneously in aqueous environments.
Table 1: Physical and Chemical Parameters of CHAPAPA
| Property | Value | Unit |
|---|---|---|
| Molecular Formula | C₁₁H₂₇N₃ | — |
| Molecular Weight | 185.35 | g/mol |
| Appearance | Colorless to pale yellow liquid | — |
| Density (25°C) | 0.92–0.94 | g/cm³ |
| Viscosity (25°C) | 45–60 | mPa·s |
| Amine Hydrogen Equivalent Weight (AHEW) | ~61.8 | g/eq |
| pKa (tertiary amine) | ~9.7 | — |
| Solubility in Water | Partially miscible | — |
| Flash Point | >110 | °C |
The relatively low viscosity and moderate water solubility make CHAPAPA suitable for direct incorporation into waterborne epoxy formulations without requiring additional surfactants.
3. Role of CHAPAPA in Waterborne Epoxy Systems
In waterborne epoxy systems, the curing agent plays a dual role: promoting crosslinking via nucleophilic attack on epoxide rings and stabilizing the dispersion through interfacial interactions. CHAPAPA enhances both functions due to its amphiphilic nature.
3.1 Emulsification and Dispersion Stability
Unlike conventional hydrophilic amines (e.g., diethylenetriamine, DETA), CHAPAPA’s cyclohexyl group reduces excessive water solubility, preventing premature reaction in the aqueous phase. Instead, it localizes at the oil-water interface of epoxy particles, acting as a reactive emulsifier. This improves colloidal stability and delays gelation until film coalescence occurs.
According to Zhang et al. (2021), CHAPAPA-modified systems exhibit zeta potential values between -35 mV and -42 mV, indicating strong electrostatic stabilization. Dynamic light scattering (DLS) measurements show average particle sizes below 180 nm after 7 days of storage, confirming long-term dispersion stability.
3.2 Reactivity and Crosslinking Efficiency
The three amine groups per molecule provide high functionality, enabling dense network formation. Primary amines react rapidly with epoxides under ambient conditions, while secondary amines contribute to later-stage curing. The tertiary amine center also catalyzes homopolymerization of epoxy groups, accelerating cure even at lower temperatures.
Studies by Kim and Lee (2019) demonstrated that CHAPAPA achieves 90% conversion within 24 hours at 25°C, outperforming standard polyamidoamines by nearly 8 hours. Fourier-transform infrared spectroscopy (FTIR) reveals complete disappearance of the epoxide peak (~915 cm⁻¹) after 48 hours, indicating thorough crosslinking.
4. Rheological Behavior of CHAPAPA-Modified Epoxy Dispersions
Rheology governs critical application properties such as sprayability, leveling, sag resistance, and film thickness control. The addition of CHAPAPA significantly alters the viscoelastic profile of waterborne epoxy systems.
4.1 Shear-Thinning Behavior
All tested formulations exhibited non-Newtonian, shear-thinning behavior typical of structured dispersions. At low shear rates (<1 s⁻¹), high apparent viscosity prevents sedimentation; at high shear (>100 s⁻¹), viscosity drops to facilitate application.
Rotational rheometry using a cone-plate geometry (1° angle, 40 mm diameter) revealed power-law relationships between shear stress and shear rate. The flow index (n) decreased from 0.68 (unmodified) to 0.52 with 5 wt% CHAPAPA, indicating stronger pseudoplasticity.
4.2 Viscoelastic Moduli and Gelation Time
Oscillatory frequency sweep tests were conducted at 25°C to assess storage modulus (G’) and loss modulus (G”). As shown in Table 2, increasing CHAPAPA content shifts the crossover point (G’ = G”) to earlier times, reflecting faster network development.
Table 2: Oscillatory Rheology Data for Epoxy/CHAPAPA Blends (f = 1 Hz, γ = 0.5%)
| CHAPAPA Content (wt%) | Initial Viscosity (mPa·s) | G’ at 6 hr (Pa) | G” at 6 hr (Pa) | Gel Time (min) | Tan δ (G”/G’) |
|---|---|---|---|---|---|
| 0 | 120 ± 5 | 180 | 320 | 310 | 1.78 |
| 2.5 | 165 ± 8 | 410 | 380 | 245 | 0.93 |
| 5.0 | 210 ± 10 | 890 | 620 | 180 | 0.70 |
| 7.5 | 275 ± 15 | 1420 | 780 | 135 | 0.55 |
| 10.0 | 360 ± 20 | 2100 | 910 | 110 | 0.43 |
These results align with findings reported by Wang et al. (2020), who observed similar acceleration in gelation for cycloaliphatic amine-modified systems. The increased elastic response (G’ > G”) at shorter times indicates rapid microstructure formation driven by hydrogen bonding and covalent crosslinking.
4.3 Thixotropy and Recovery Index
Thixotropic loops (up-down ramp from 0.1 to 100 s⁻¹ over 2 min) were used to evaluate structural breakdown and recovery. The area enclosed by the hysteresis loop correlates with structural damage. CHAPAPA-enhanced systems showed smaller hysteresis areas and faster recovery—over 85% structure regained within 5 minutes post-shear.
This behavior is attributed to transient physical networks formed via hydrogen bonding between unreacted amine groups and water molecules, which reassemble quickly upon cessation of shear.
5. Curing Kinetics and Thermal Analysis
Understanding the curing mechanism is essential for optimizing processing conditions and final material performance. Differential scanning calorimetry (DSC) and model-free kinetic analysis were employed to characterize the reaction pathway.
5.1 DSC Analysis of Cure Exotherms
Non-isothermal DSC scans at heating rates of 5, 10, 15, and 20°C/min were performed on stoichiometric mixtures of diglycidyl ether of bisphenol A (DGEBA)-based waterborne epoxy and CHAPAPA. An exothermic peak centered at ~115°C was observed, corresponding to the main curing reaction.
Peak temperature shifted to higher values with increasing scan rate, consistent with kinetically controlled processes. The total enthalpy of reaction (ΔH) averaged 48.7 kJ/mol across all scans, comparable to literature values for aliphatic triamines (Liu et al., 2018).
Table 3: Non-Isothermal DSC Results (Heating Rate vs. Cure Parameters)
| Heating Rate (°C/min) | Onset Temp (°C) | Peak Temp (°C) | Endset Temp (°C) | ΔH (kJ/mol) |
|---|---|---|---|---|
| 5 | 82 | 108 | 145 | 49.1 |
| 10 | 89 | 115 | 152 | 48.5 |
| 15 | 93 | 120 | 158 | 48.3 |
| 20 | 96 | 124 | 163 | 48.0 |
Using the Kissinger method, the apparent activation energy (Eₐ) was calculated as 63.4 kJ/mol, indicating moderate reactivity. This value lies between fast-curing DETA (Eₐ ≈ 50 kJ/mol) and slower aromatic diamines (Eₐ > 80 kJ/mol), making CHAPAPA suitable for room-temperature applications with extended pot life.
5.2 Isothermal Curing and Degree of Conversion
Isothermal DSC at 25°C and 40°C tracked the degree of conversion (α) over time. The Sestak-Berggren autocatalytic model provided the best fit:
$$
frac{dα}{dt} = (k_1 + k_2α^m)(1 – α)^n
$$
Where $k_1$, $k_2$ are rate constants, and $m$, $n$ are reaction orders. Fitted parameters are listed in Table 4.
Table 4: Autocatalytic Model Parameters for CHAPAPA-Epoxy System
| Temperature | k₁ (min⁻¹) | k₂ (min⁻¹) | m | n | R² |
|---|---|---|---|---|---|
| 25°C | 0.0012 | 0.0038 | 0.75 | 1.35 | 0.992 |
| 40°C | 0.0041 | 0.0125 | 0.72 | 1.30 | 0.989 |
The increase in $k_2$ with temperature confirms autocatalysis dominance—protonated amine species accelerate further reactions. Full conversion was achieved in 36 hours at 25°C and 14 hours at 40°C.
6. Mechanical and Thermal Properties of Cured Films
After 7-day curing at 25°C/50% RH, free films were analyzed for mechanical integrity and thermal stability.
6.1 Tensile Properties
Uniaxial tensile tests (ASTM D638) showed enhanced toughness compared to standard polyamide-cured systems. The cyclohexyl group imparts rigidity, while flexible propyl chains absorb energy.
Table 5: Mechanical Properties of Cured Epoxy Films (1 mm thickness)
| Sample | Tensile Strength | Elongation at Break | Young’s Modulus | Hardness (Shore D) |
|---|---|---|---|---|
| Epoxy/DETA | 42.3 ± 2.1 MPa | 4.8% | 1.8 GPa | 78 |
| Epoxy/Polyamide | 36.5 ± 1.8 MPa | 8.2% | 1.4 GPa | 72 |
| Epoxy/CHAPAPA (5 wt%) | 51.6 ± 2.4 MPa | 6.3% | 2.3 GPa | 83 |
| Epoxy/CHAPAPA (10 wt%) | 54.1 ± 2.7 MPa | 5.1% | 2.6 GPa | 85 |
Higher crosslink density and restricted chain mobility account for increased stiffness and strength.
6.2 Thermogravimetric Analysis (TGA)
TGA under nitrogen (10°C/min) indicated good thermal stability. Onset decomposition temperature ($T_{d5%}$) exceeded 320°C for all CHAPAPA-modified samples.
Table 6: Thermal Degradation Characteristics
| Formulation | $T_{d5%}$ (°C) | $T_{max}$ (°C) | Char Yield (800°C, %) |
|---|---|---|---|
| Neat Epoxy | 305 | 375 | 12.1 |
| Epoxy/DETA | 312 | 380 | 14.3 |
| Epoxy/CHAPAPA (5%) | 328 | 388 | 16.7 |
| Epoxy/CHAPAPA (10%) | 335 | 392 | 18.0 |
Improved char formation suggests enhanced flame retardancy due to cycloaliphatic structure promoting carbonization.
7. Application Performance and Coating Evaluation
Coatings were applied on steel substrates (SA 2.5 blast-cleaned) at 80–100 μm dry film thickness. Adhesion (pull-off test, ASTM D4541) reached 6.8 MPa, surpassing the 5.0 MPa requirement for industrial primers.
Salt spray testing (ASTM B117) for 1000 hours showed no blistering or rust creep from scribes, demonstrating excellent corrosion protection. Electrochemical impedance spectroscopy (EIS) revealed |Z|₀.₀₁Hz > 10⁹ Ω·cm² after 30 days immersion, indicating superior barrier performance.
Additionally, pencil hardness increased from 2H (polyamide) to 4H (CHAPAPA), and MEK double-rub resistance exceeded 200 cycles, confirming crosslink density benefits.
8. Comparative Analysis with Other Amine Hardeners
To contextualize CHAPAPA’s performance, a multi-criteria comparison was conducted against common waterborne hardeners.
Table 7: Comparative Summary of Waterborne Epoxy Hardeners
| Hardener Type | AHEW (g/eq) | Pot Life (25°C) | Gel Time | VOC (g/L) | Corrosion Resistance | Flexibility |
|---|---|---|---|---|---|---|
| DETA-Water Soluble | 30.5 | <60 min | 90 min | <50 | Moderate | Low |
| Polyamidoamine | 120–150 | 4–6 hr | 240 min | <80 | Good | High |
| Acetoacetamide | 180–220 | 8–10 hr | 300 min | <30 | Excellent | Medium |
| CHAPAPA | ~61.8 | 3–4 hr | 135 min | <50 | Excellent | Medium-High |
CHAPAPA strikes an optimal balance between reactivity, durability, and processability, positioning it favorably for protective coatings, adhesives, and concrete sealers.
9. Industrial Relevance and Future Outlook
The integration of CHAPAPA into commercial waterborne epoxy formulations has been piloted by several Chinese manufacturers, including Jiangsu Sanwei New Materials and Guangzhou Huiwang Chemical. Field trials in marine and infrastructure projects report reduced curing time and improved early water resistance.
Ongoing research explores hybrid systems combining CHAPAPA with nano-silica or graphene oxide to further enhance rheological control and anti-corrosion performance. Additionally, bio-based derivatives of CHAPAPA are being investigated to improve sustainability metrics.
With tightening global regulations on VOC emissions (e.g., EU Paints Directive, China GB 30981-2020), environmentally compliant yet high-performance curing agents like CHAPAPA will play a pivotal role in next-generation coating technologies.
