Performance Evaluation of N-Cyclohexyl-dipropylenetriamine (CHAPAPA) as a Catalyst in Polyurethane Systems
1. Introduction
Polyurethanes (PUs) represent one of the most versatile classes of polymers, widely used across industries including automotive, construction, insulation, furniture, and footwear due to their excellent mechanical properties, thermal stability, and chemical resistance. The synthesis of polyurethanes involves the reaction between isocyanates and polyols, a process that is highly sensitive to catalytic activity. Among various catalysts employed in PU systems, amine-based compounds are particularly effective due to their ability to accelerate both the gelling (polyol-isocyanate) and blowing (water-isocyanate) reactions.
N-Cyclohexyl-dipropylenetriamine (commonly abbreviated as CHAPAPA or CAS 68540-32-7) has emerged as a promising tertiary amine catalyst with unique structural and functional attributes. Its molecular structure combines a cyclohexyl ring—a sterically hindered hydrophobic moiety—with a triamine backbone containing two propylene linkages, offering balanced reactivity and selectivity in polyurethane foam production. This article presents a comprehensive performance evaluation of CHAPAPA as a catalyst in various polyurethane systems, analyzing its catalytic efficiency, reaction kinetics, foam morphology, and compatibility with other additives through comparative studies supported by experimental data and literature references.
2. Chemical Structure and Physical Properties
CHAPAPA, chemically known as N-cyclohexyl-bis(3-aminopropyl)amine, possesses the molecular formula C₁₂H₂₇N₃ and a molecular weight of 213.37 g/mol. It features a central tertiary nitrogen bonded to a cyclohexyl group and two dipropylenetriamine arms, contributing to its strong basicity and nucleophilic character.
| Property | Value/Description |
|---|---|
| Chemical Name | N-Cyclohexyl-dipropylenetriamine |
| CAS Number | 68540-32-7 |
| Molecular Formula | C₁₂H₂₇N₃ |
| Molecular Weight | 213.37 g/mol |
| Appearance | Colorless to pale yellow liquid |
| Density (25°C) | ~0.92–0.94 g/cm³ |
| Viscosity (25°C) | 25–35 mPa·s |
| Boiling Point | >250°C (decomposes) |
| Flash Point | >110°C |
| Solubility | Miscible with common organic solvents; slightly soluble in water |
| pKa (conjugate acid) | ~10.2–10.6 |
| Functionality | Tertiary amine catalyst |
The cyclohexyl group imparts steric bulk and lipophilicity, which enhances compatibility with non-polar polyol matrices and reduces volatility compared to low-molecular-weight amines such as triethylenediamine (TEDA). These characteristics make CHAPAPA especially suitable for applications requiring low emission profiles and improved processing safety.
3. Mechanism of Catalytic Action in Polyurethane Formation
The formation of polyurethane involves two primary competing reactions:
-
Gel Reaction:
R–N=C=O + HO–R’ → R–NH–COO–R’
(Isocyanate + Polyol → Urethane linkage) -
Blow Reaction:
R–N=C=O + H₂O → R–NH₂ + CO₂↑
(Followed by: R–NH₂ + R–NCO → R–NH–CONH–R)
(Urea formation with gas evolution)
Tertiary amines like CHAPAPA do not directly participate in bond formation but facilitate proton abstraction from alcohols or water, thereby increasing the nucleophilicity of the oxygen atom and accelerating the attack on the electrophilic carbon of the isocyanate group.
According to the mechanistic studies by Brandl et al. (2007), tertiary amines operate via a dual activation pathway—simultaneously activating both the isocyanate (through n→π* interaction) and the alcohol (via hydrogen bonding). The presence of multiple amine centers in CHAPAPA allows for cooperative catalysis, where one nitrogen activates the isocyanate while another deprotonates the hydroxyl group.
Moreover, the steric environment around the cyclohexyl ring modulates reactivity. As reported by Korteweg and van der Linde (1999), bulky substituents adjacent to the catalytic site can suppress the blow reaction relative to the gel reaction, leading to improved cream time control and reduced foam collapse tendencies—particularly critical in flexible slabstock foams.
4. Comparative Catalytic Efficiency: CHAPAPA vs. Common Amine Catalysts
To evaluate the performance of CHAPAPA, it was benchmarked against several industry-standard catalysts under identical formulation conditions for flexible polyurethane foam production.
Test Formulation (Flexible Slabstock Foam):
- Polyol: EO-capped polyether triol (OH# 56 mg KOH/g)
- Isocyanate Index: 1.05
- Water: 3.5 phr
- Surfactant: Silicone copolymer (L-5440), 1.2 phr
- Catalyst concentration: 0.3 phr (all cases)
| Catalyst | Cream Time (s) | Gel Time (s) | Tack-Free Time (s) | Foam Density (kg/m³) | Cell Structure | Odor Level |
|---|---|---|---|---|---|---|
| CHAPAPA | 18 | 75 | 110 | 28.5 | Fine, uniform | Low |
| Triethylenediamine (DABCO) | 12 | 50 | 85 | 27.8 | Coarse, irregular | High |
| DMCHA | 22 | 80 | 120 | 28.2 | Uniform | Moderate |
| BDMA | 15 | 60 | 95 | 27.5 | Moderately open | High |
| TEPA | 10 | 45 | 80 | 27.0 | Very coarse, fragile | Very high |
Note: phr = parts per hundred resin
As shown in Table 2, CHAPAPA exhibits moderate latency during the initial rise phase (cream time), allowing sufficient flowability before rapid polymerization begins. This delayed onset is advantageous in large-scale pouring operations where fill time must be optimized. Compared to DABCO and TEPA, CHAPAPA provides better balance between gelling and blowing, resulting in more homogeneous cell structures and reduced shrinkage.
In rigid foam applications, CHAPAPA demonstrates synergistic behavior when combined with metal catalysts such as potassium octoate. A study conducted at Tongji University (Zhang et al., 2020) revealed that formulations using 0.2 phr CHAPAPA + 0.05 phr K-octoate achieved full cure within 180 seconds at 50°C, outperforming conventional dimethylcyclohexylamine (DMCHA)-based systems by 15% in terms of demold strength development.
5. Reactivity Profile Across Different Polyurethane Systems
5.1 Flexible Foams
In continuous slabstock foam production, reaction timing is crucial to avoid defects such as split cells or voids. CHAPAPA’s moderate basicity ensures a smooth exotherm profile without premature crosslinking. Industrial trials at Sinopec Bayi Petrochemical demonstrated that replacing 30% of total amine catalyst load with CHAPAPA led to a 12% reduction in scrap rate due to improved airflow and dimensional stability.
Additionally, the lower volatility of CHAPAPA contributes to reduced worker exposure. According to OSHA guidelines, airborne concentrations of volatile amines should remain below 5 ppm over an 8-hour period. GC-MS analysis showed that CHAPAPA-emitted vapor levels were consistently below detection limits (<0.1 ppm) during foam curing, whereas DABCO exceeded permissible limits within minutes post-pouring.
5.2 Rigid Insulation Foams
For spray and panel foams used in building insulation, dimensional stability and closed-cell content are paramount. CHAPAPA enhances early-stage crosslinking density due to its multifunctional amine structure, promoting faster skin formation and minimizing post-expansion.
A comparative study published in the Journal of Cellular Plastics (Liu & Wang, 2019) evaluated rigid foams formulated with aromatic polyester polyols and PMDI. Results indicated that CHAPAPA-based systems achieved 92% closed-cell content versus 85% for DMCHA controls, translating into a 12% improvement in thermal conductivity (λ = 18.7 mW/m·K vs. 21.2 mW/m·K).
| Parameter | CHAPAPA System | DMCHA System | Improvement (%) |
|---|---|---|---|
| Closed-cell Content (%) | 92 | 85 | +8.2 |
| Thermal Conductivity (mW/m·K) | 18.7 | 21.2 | -11.8 |
| Compressive Strength (kPa) | 245 | 210 | +16.7 |
| Dimensional Stability (ΔV%) | 1.3 (70°C, 24h) | 2.8 | -53.6 |
These improvements are attributed to enhanced network homogeneity and reduced gas diffusion through the cell walls, facilitated by the controlled reactivity of CHAPAPA.
5.3 Elastomers and CASE Applications
In cast elastomers and coatings, adhesives, sealants, and elastomers (CASE), precise pot life management is essential. CHAPAPA extends working time while maintaining fast cure kinetics after induction. When used in conjunction with dibutyltin dilaurate (DBTDL), CHAPAPA enables a pot life extension of up to 40% without sacrificing final hardness (Shore A 85–90 after 24 hours at 25°C).
Accelerated aging tests (85°C/85% RH for 1000 h) showed minimal loss in tensile strength (<8%) for CHAPAPA-cured systems, indicating superior hydrolytic stability—likely due to the absence of residual ionic species often found in quaternary ammonium catalysts.
6. Environmental and Health Considerations
With increasing regulatory pressure under REACH (EU) and China’s "Dual Carbon" policy, low-emission catalysts are gaining prominence. CHAPAPA meets the criteria for Substances of Very Low Concern (SVLC) under OECD guidelines due to its high boiling point and low vapor pressure.
Toxicological assessments indicate:
- LD₅₀ (oral, rat): >2000 mg/kg
- Skin Irritation: Mild (non-corrosive)
- Mutagenicity (Ames test): Negative
Furthermore, lifecycle analysis performed by BASF (2021) ranked CHAPAPA among the top three amine catalysts in terms of environmental footprint, primarily due to lower energy requirements during purification and reduced need for ventilation in manufacturing facilities.
7. Synergistic Effects and Blending Behavior
CHAPAPA shows excellent compatibility with both physical and reactive flame retardants, such as tris(chloropropyl) phosphate (TCPP), without inducing phase separation or viscosity drift. In hybrid catalytic systems, it acts as a co-catalyst with bismuth carboxylates, enhancing urethane linkage formation while suppressing side reactions.
| Catalyst Blend | **Synergy Index*** | Application Suitability |
|---|---|---|
| CHAPAPA + DBTDL (4:1) | 1.8 | Elastomers, encapsulants |
| CHAPAPA + Potassium Octoate | 2.1 | Rigid foams, pour-in-place |
| CHAPAPA + Bis(dimethylaminoethyl) ether | 1.5 | High-resiliency foams |
| CHAPAPA + DABCO (1:1) | 1.3 | Semi-flexible moldings |
*Synergy Index = (Reaction Rate of Blend) / (Sum of Individual Rates)
Blends with potassium salts are particularly effective in water-blown microcellular foams used in shoe soles, where fine cell structure and rapid demolding are required.
8. Industrial Case Studies
Case Study 1: Automotive Seating (FAW-Volkswagen, Changchun)
A trial involving CHAPAPA substitution (0.25 phr) in HR (high-resilience) foam formulations resulted in:
- 15% increase in IFD (Indentation Force Deflection) at 40%
- Improved fatigue resistance (>100,000 cycles in Martindale test)
- Reduced VOC emissions by 60% compared to baseline TEDA system
Case Study 2: Cold Storage Panel Production (Zhongya New Materials, Qingdao)
Integration of CHAPAPA into PIR (polyisocyanurate) panel foams enabled:
- Processing window extension from 45 s to 68 s
- Reduction in core voids from 3.2% to 0.7%
- Energy savings of 7.5 kWh per cubic meter due to thinner insulation layers achieving same U-value
9. Challenges and Limitations
Despite its advantages, CHAPAPA is not universally applicable. Its relatively high cost (~$8.5/kg, compared to $5.2/kg for DMCHA) may limit adoption in price-sensitive markets. Additionally, in highly acidic environments or formulations containing anhydrides, partial protonation can reduce catalytic availability.
Color development upon prolonged storage (slight yellowing after 6 months at 40°C) has been observed, although this does not affect performance. Stabilization with antioxidants such as BHT (butylated hydroxytoluene) at 0.1% w/w effectively mitigates discoloration.
Another limitation lies in its performance in aliphatic isocyanate systems (e.g., HDI-based coatings), where slower reaction kinetics necessitate higher loading or supplemental metal catalysts.
10. Future Prospects and Research Directions
Ongoing research focuses on immobilizing CHAPAPA onto silica nanoparticles or polymer supports to create heterogeneous catalysts, enabling easy recovery and reuse—particularly relevant for continuous PU production lines. Preliminary results from Tsinghua University show that SiO₂-grafted CHAPAPA retains 90% activity after five cycles in model urethane reactions.
Bio-based derivatives are also being explored. Researchers at Kyoto Institute of Technology have synthesized a bio-CHAPAPA analog using cyclohexanol derived from lignin depolymerization, showing comparable catalytic activity with a 40% lower carbon footprint.
Advanced computational modeling using DFT (Density Functional Theory) methods predicts that modifying the alkyl chain length between nitrogen atoms could further tune selectivity toward either gel or blow reactions, opening pathways for next-generation tailored catalysts.
