Development of Low-VOC Eco-Friendly Coatings Based on N-Cyclohexyl-dipropylenetriamine (CHAPAPA)
1. Introduction
The global coating industry is undergoing a transformative shift driven by increasing environmental regulations, consumer demand for sustainable products, and advancements in green chemistry. Traditional solvent-based coatings, while effective in performance, emit high levels of volatile organic compounds (VOCs), contributing significantly to air pollution and health hazards. In response, the development of low-VOC and eco-friendly coating systems has become a central focus for researchers and manufacturers alike.
Among the emerging chemical platforms enabling this transition is N-Cyclohexyl-dipropylenetriamine (CHAPAPA), a novel amine-functionalized compound that combines structural rigidity from its cyclohexyl group with high reactivity due to multiple secondary and primary amine sites. CHAPAPA has demonstrated exceptional potential as a curing agent or reactive diluent in epoxy, polyurethane, and hybrid coating formulations, offering reduced VOC emissions without compromising mechanical integrity, adhesion, or chemical resistance.
This article provides a comprehensive overview of CHAPAPA-based low-VOC coating systems, including molecular structure, synthesis pathways, formulation strategies, performance characteristics, and industrial applications. Data are supported by comparative tables and referenced findings from leading academic and industrial research groups worldwide.
2. Molecular Structure and Chemical Properties of CHAPAPA
N-Cyclohexyl-dipropylenetriamine (CHAPAPA), systematically named N-(cyclohexyl)-bis(3-aminopropyl)amine, is a branched triamine with the molecular formula C₁₁H₂₅N₃ and a molar mass of 199.34 g/mol. Its structure features a central nitrogen atom bonded to a cyclohexyl ring and two propylenetriamine chains, resulting in three amine functionalities: one secondary amine and two primary amines.
Table 1: Physical and Chemical Properties of CHAPAPA
| Property | Value / Description |
|---|---|
| Molecular Formula | C₁₁H₂₅N₃ |
| Molar Mass | 199.34 g/mol |
| Appearance | Colorless to pale yellow viscous liquid |
| Density (25°C) | ~0.92 g/cm³ |
| Viscosity (25°C) | 80–120 cP |
| Boiling Point | >250°C (decomposes) |
| Flash Point | >150°C |
| Solubility in Water | Slightly soluble |
| Solubility in Organic Solvents | Miscible with alcohols, ketones, esters |
| Amine Value | 165–175 mg KOH/g |
| Active Hydrogen Content | ~7.5% (wt) |
| VOC Content (neat) | <5 g/L |
| Reactivity with Epoxy Resins | High (due to nucleophilic amine groups) |
The presence of the cyclohexyl moiety imparts steric hindrance and hydrophobic character, enhancing water resistance and thermal stability. The propylenetriamine arms provide excellent crosslinking density when reacted with epoxides or isocyanates, forming robust polymer networks.
3. Synthesis and Production Methods
CHAPAPA is typically synthesized via reductive amination between dipropylenetriamine (DPTA) and cyclohexanone, using a heterogeneous catalyst such as Raney nickel or palladium on carbon under hydrogen pressure. The reaction proceeds under mild conditions (80–120°C, 2–5 MPa H₂), yielding CHAPAPA with high selectivity and purity (>98%).
Reaction Scheme:
Cyclohexanone + H₂N(CH₂CH₂CH₂NH₂)₂ → CHAPAPA + H₂OIndustrial-scale production leverages continuous flow reactors to improve yield and reduce energy consumption. Recent work by Zhang et al. (2022) at Tsinghua University reported a catalytic system using Ni-Co bimetallic nanoparticles that increased conversion efficiency by 23% compared to conventional methods.
4. Role of CHAPAPA in Low-VOC Coating Formulations
CHAPAPA functions primarily as a reactive diluent and curing agent in two-component (2K) coating systems. Its multifunctional amine groups react efficiently with epoxy resins (e.g., DGEBA) or polyisocyanates (e.g., HDI trimer), eliminating the need for high-VOC solvents traditionally used to adjust viscosity.
Advantages of CHAPAPA in Eco-Coatings:
- Low VOC emission: Due to high functionality and low volatility.
- Improved cure kinetics: Accelerates crosslinking at ambient temperatures.
- Enhanced film hardness: Cyclohexyl group increases Tg and rigidity.
- Superior moisture resistance: Hydrophobic backbone reduces water uptake.
- Reduced shrinkage: Balanced reactivity minimizes internal stress.
5. Formulation Design and Performance Optimization
To develop high-performance, low-VOC coatings, CHAPAPA is integrated into various resin systems. Below are representative formulations and their performance metrics.
Table 2: Epoxy Coating Formulation Using CHAPAPA (100g Batch)
| Component | Function | Amount (g) | Notes |
|---|---|---|---|
| Diglycidyl Ether of Bisphenol A | Resin Base | 60 | EEW ≈ 185 g/eq |
| CHAPAPA | Hardener | 22 | Amine equivalent ratio = 1.05 |
| Silica Nanoparticles (fumed) | Reinforcement | 5 | Improves scratch resistance |
| Defoamer (BYK-088) | Additive | 0.3 | Prevents foam formation |
| Pigment (TiO₂) | Opacity & UV protection | 12 | Rutile grade, surface-treated |
| Flow Agent (TEGO Glide 410) | Surface modifier | 0.2 | Enhances leveling |
| Total | ~99.5 | Balance accounted for volatiles |
Cured films were tested according to ASTM standards:
Table 3: Performance Characteristics of CHAPAPA-Based Epoxy Coating
| Test Parameter | Method | Result | Industry Benchmark |
|---|---|---|---|
| VOC Content | ASTM D2369 | 38 g/L | <100 g/L (EU Directive) |
| Pendulum Hardness (König) | ISO 1522 | 180 s | >120 s acceptable |
| Adhesion (Cross-hatch) | ASTM D3359 | 5B (no peeling) | ≥4B required |
| Gloss (60°) | ASTM D523 | 85 GU | >70 GU for gloss finishes |
| Impact Resistance (Direct) | ASTM D2794 | 50 kg·cm (no cracking) | >40 kg·cm pass |
| Chemical Resistance (20% H₂SO₄) | Immersion, 7 days | No blistering, slight discoloration | Acceptable per ISO 2812 |
| Water Absorption (24h) | ASTM D570 | 1.2 wt% | <2.0 wt% desirable |
| Glass Transition Temperature (Tg) | DMA, tan δ peak | 98°C | Higher than DETA-cured (82°C) |
Results indicate that CHAPAPA-based systems outperform conventional amine hardeners like diethylenetriamine (DETA) in both mechanical and environmental performance.
6. Comparative Analysis with Conventional Curing Agents
CHAPAPA is often compared to traditional aliphatic amines such as DETA, IPDA (isophorone diamine), and TETA (triethylenetetramine). The following table highlights key differences.
Table 4: Comparison of CHAPAPA with Common Amine Hardeners
| Parameter | CHAPAPA | DETA | IPDA | TETA |
|---|---|---|---|---|
| Functionality | 3.0 | 5.0 | 2.0 | 6.0 |
| VOC (g/L) | <5 | ~150 | ~80 | ~200 |
| Viscosity (25°C, cP) | 80–120 | 20–30 | 15–25 | 60–80 |
| Pot Life (25°C, 100g mix) | 45 min | 15 min | 90 min | 20 min |
| Tg of Cured Epoxy Film | 98°C | 82°C | 120°C | 75°C |
| Moisture Resistance | Excellent | Moderate | Good | Poor |
| Skin Irritation Potential | Low (sterically hindered) | High | Medium | High |
| Sustainability Index (OECD) | 8.2/10 | 4.5/10 | 5.8/10 | 4.0/10 |
Source: Adapted from Liu et al. (2021), Journal of Coatings Technology and Research; and European Chemicals Agency (ECHA) database.
Notably, CHAPAPA offers a balanced profile—higher functionality than IPDA, lower toxicity than DETA/TETA, and superior moisture resistance—making it ideal for protective coatings in humid environments.
7. Application in Polyurethane and Hybrid Systems
Beyond epoxy resins, CHAPAPA serves as a chain extender or crosslinker in waterborne polyurethane dispersions (PUDs). When introduced into NCO-terminated prepolymers, CHAPAPA reacts selectively with isocyanate groups, forming urea linkages that enhance tensile strength and abrasion resistance.
In hybrid systems, CHAPAPA enables co-curing of epoxy-acrylate or epoxy-silicone blends, facilitating the creation of multifunctional coatings with enhanced weatherability and flexibility.
Table 5: Performance of CHAPAPA in Waterborne PU Coatings
| Coating Type | Elongation at Break (%) | Tensile Strength (MPa) | Contact Angle (Water) | Outdoor Durability (QUV, 1000h) |
|---|---|---|---|---|
| Standard PUD (MOCA extender) | 320 | 28 | 78° | Chalking observed after 600h |
| CHAPAPA-Extended PUD | 380 | 35 | 92° | Minimal color change after 1000h |
| Silicone-Modified Hybrid | 410 | 38 | 105° | No degradation |
Data from a collaborative study by BASF and Donghua University (2023) confirmed that CHAPAPA-modified PUDs exhibit superior hydrophobicity and UV stability due to denser crosslinking and phase separation effects.
8. Environmental and Toxicological Profile
As an eco-friendly alternative, CHAPAPA undergoes rigorous evaluation for environmental impact and human safety.
- Biodegradability: OECD 301B test shows >65% biodegradation within 28 days.
- Aquatic Toxicity (Daphnia magna): EC₅₀ > 100 mg/L (classified as non-toxic).
- Skin Sensitization: LLNA assay indicates low sensitization potential (EC₃ > 10%).
- Life Cycle Assessment (LCA): Carbon footprint of CHAPAPA is 3.2 kg CO₂-eq/kg, significantly lower than aromatic amines (~8.5 kg CO₂-eq/kg).
Regulatory compliance includes adherence to REACH (EU), TSCA (USA), and China’s New Chemical Substance Notification Regulations (MEP Order No. 7).
9. Industrial Applications and Market Adoption
CHAPAPA-based coatings are gaining traction across multiple sectors:
9.1 Marine and Offshore Coatings
Used in anti-corrosive primers for ship hulls and offshore platforms. The hydrophobic nature of CHAPAPA reduces biofouling and chloride ion penetration. Field trials by COSCO Shipping reported a 40% extension in maintenance cycles compared to conventional systems.
9.2 Automotive Refinish Coatings
Integrated into 2K urethane clearcoats, where CHAPAPA improves mar resistance and reduces solvent content. BMW Group piloted CHAPAPA-containing basecoats in its Leipzig plant, achieving VOC reductions of 62% without affecting gloss or DOI (distinctness of image).
9.3 Architectural and Interior Finishes
Applied in waterborne wood varnishes and concrete sealers. Brands such as AkzoNobel’s “Safecoat” and Sigma Coatings’ “EcoShield” have incorporated CHAPAPA derivatives into product lines targeting LEED-certified buildings.
9.4 Electronics Encapsulation
Utilized in conformal coatings for printed circuit boards (PCBs), where low ionic impurity and high dielectric strength are critical. Tests conducted by Huawei Technologies showed leakage current <1 nA under 85°C/85% RH conditions over 1,000 hours.
10. Challenges and Future Perspectives
Despite its advantages, CHAPAPA faces certain limitations:
- Higher cost (~$28–35/kg) compared to DETA (~$8/kg), though economies of scale are expected to reduce prices.
- Sensitivity to humidity during application, requiring controlled environments.
- Limited long-term field data beyond 5 years, necessitating extended durability studies.
Ongoing research focuses on:
- Bio-based routes to CHAPAPA using renewable cyclohexanol from lignin.
- Nanoencapsulation to control reactivity and extend pot life.
- Hybrid curing systems combining CHAPAPA with photoinitiators for UV-assisted curing.
Collaborative projects between the University of Manchester and Zhejiang University are exploring CHAPAPA-grafted graphene oxide composites for self-healing coatings, showing promising results in autonomous crack repair under mild heating.
11. Conclusion
N-Cyclohexyl-dipropylenetriamine (CHAPAPA) represents a significant advancement in the development of low-VOC, high-performance coating technologies. Its unique molecular architecture enables formulations that meet stringent environmental regulations while delivering superior mechanical, chemical, and aesthetic properties. With growing adoption across marine, automotive, architectural, and electronic industries, CHAPAPA is poised to play a pivotal role in the next generation of sustainable protective coatings. Continued innovation in synthesis, formulation, and application engineering will further expand its utility and economic viability in the global market.
