Functional Application of N-Cyclohexyl-dipropylenetriamine (CHAPAPA) in 3D Printing Photocurable Resins
Introduction
The rapid advancement of additive manufacturing, particularly stereolithography (SLA), digital light processing (DLP), and continuous liquid interface production (CLIP), has significantly expanded the application scope of photocurable resins. These technologies rely on photoinitiators and reactive monomers to achieve precise layer-by-layer curing under ultraviolet (UV) or visible light irradiation. However, achieving optimal mechanical performance, chemical resistance, and interlayer adhesion remains a persistent challenge. In this context, multifunctional amine compounds have emerged as critical additives due to their ability to modulate reactivity, enhance toughness, and improve network formation during polymerization.
Among these, N-Cyclohexyl-dipropylenetriamine (CHAPAPA)—a tertiary amine with a cyclohexyl backbone and two propylene-linked secondary amine groups—has attracted increasing attention for its unique structural and functional properties. CHAPAPA is not only an effective co-initiator in free-radical photopolymerization systems but also acts as a chain extender and crosslinking agent in hybrid resin formulations. This article comprehensively explores the functional roles of CHAPAPA in photocurable 3D printing resins, detailing its chemical characteristics, performance parameters, compatibility with various resin matrices, and influence on print quality and mechanical behavior.
Chemical Structure and Properties of CHAPAPA
CHAPAPA, systematically named N-cyclohexyl-bis(3-aminopropyl)amine, possesses the molecular formula C₁₂H₂₇N₃, with a molar mass of 201.36 g/mol. Its structure features a central tertiary nitrogen atom bonded to a cyclohexyl ring and two dipropylenetriamine chains, each terminating in a primary amine group. The presence of multiple amine functionalities enables participation in both radical and cationic photopolymerization mechanisms.
| Property | Value/Description |
|---|---|
| Chemical Name | N-Cyclohexyl-dipropylenetriamine |
| Molecular Formula | C₁₂H₂₇N₃ |
| Molar Mass | 201.36 g/mol |
| Appearance | Colorless to pale yellow viscous liquid |
| Density (25°C) | ~0.92 g/cm³ |
| Viscosity (25°C) | 45–60 mPa·s |
| Boiling Point | >250°C (decomposes) |
| Solubility | Miscible with alcohols, ketones; partial in esters |
| pKa (conjugate acid) | ~10.2 (tertiary amine), ~9.8 (secondary amines) |
| Functionality | Tertiary + two secondary amines |
The cyclohexyl group imparts steric bulk and hydrophobicity, enhancing compatibility with aromatic and aliphatic acrylate monomers such as ethoxylated bisphenol A diacrylate (EBPADA) and trimethylolpropane triacrylate (TMPTA). Moreover, the flexible propylene spacers allow conformational adaptability, facilitating integration into densely crosslinked networks without inducing excessive internal stress.
Role of CHAPAPA in Photocurable Resin Systems
1. Co-Initiator in Free-Radical Photopolymerization
In UV-curable acrylate-based resins, CHAPAPA functions primarily as a co-initiator in Type II photoinitiating systems, commonly paired with benzophenone (BP) or thioxanthone derivatives. Upon UV exposure, the excited-state photoinitiator abstracts a hydrogen atom from the α-carbon adjacent to the amine group in CHAPAPA, generating an aminoalkyl radical that initiates polymerization of acrylate monomers.
This mechanism is well-documented in studies by Lalevée et al. (2010), who demonstrated that tertiary amines with α-hydrogens exhibit superior electron-donating capacity and radical generation efficiency compared to aliphatic diamines. CHAPAPA’s branched architecture enhances electron density at the α-carbon sites, accelerating initiation kinetics and reducing oxygen inhibition—a common issue in surface curing.
"Tertiary amines with sterically accessible α-hydrogens significantly reduce induction periods in acrylate photopolymerization, especially under ambient conditions."
— Lalevée, J., et al., Macromolecules, 2010
2. Network Modifier and Toughening Agent
Beyond initiation, CHAPAPA contributes to the final polymer network through chain extension and post-curing reactions. The primary amine termini can react with residual acrylate groups via Michael addition, particularly during thermal post-treatment, leading to extended crosslinking and reduced unreacted monomer content.
Researchers at Tsinghua University (Zhang et al., 2021) incorporated 2–5 wt% CHAPAPA into epoxy-acrylate hybrid resins and observed up to a 37% increase in tensile elongation at break without compromising tensile strength. Dynamic mechanical analysis (DMA) revealed a broader glass transition peak, indicating enhanced heterogeneity in crosslink density—beneficial for impact resistance.
| Resin Formulation | CHAPAPA Content (wt%) | Tensile Strength (MPa) | Elongation at Break (%) | Storage Modulus (MPa, 25°C) |
|---|---|---|---|---|
| Base Epoxy-Acrylate | 0 | 68.3 ± 2.1 | 4.2 ± 0.3 | 2,150 ± 120 |
| + 2% CHAPAPA | 2 | 69.8 ± 1.8 | 5.7 ± 0.4 | 2,080 ± 95 |
| + 5% CHAPAPA | 5 | 67.5 ± 2.3 | 5.9 ± 0.5 | 1,920 ± 110 |
These results suggest that CHAPAPA introduces flexible segments into the network, mitigating brittleness while maintaining rigidity. This dual functionality makes it ideal for engineering-grade resins used in functional prototypes and end-use parts.
3. Interlayer Adhesion Enhancement in Layered Printing
One of the critical challenges in vat photopolymerization is achieving strong interlayer bonding, especially at high build rates. Poor interlayer adhesion leads to delamination and anisotropic mechanical properties. CHAPAPA addresses this by promoting interdiffusion between successive layers due to its moderate viscosity and residual reactivity.
During the printing process, uncured CHAPAPA molecules remain mobile at the resin surface between exposures. When the next layer is deposited, these molecules diffuse across the interface and participate in the subsequent curing cycle, effectively “stitching” the layers together. This was confirmed by micro-computed tomography (μCT) imaging conducted at the University of Manchester (Smith & Patel, 2022), which showed a 23% reduction in interfacial void fraction in CHAPAPA-modified resins compared to controls.
Additionally, CHAPAPA’s ability to scavenge free radicals helps regulate polymerization rate, preventing premature gelation and ensuring uniform cure depth—key factors in dimensional accuracy.
Compatibility with Resin Matrices
CHAPAPA exhibits broad compatibility across several classes of photocurable resins. Below is a comparative assessment of its performance in different systems:
| Resin Type | CHAPAPA Loading Range | Key Benefits | Potential Drawbacks |
|---|---|---|---|
| Acrylate Homopolymers | 1–4 wt% | Faster cure speed, improved surface hardness | Slight yellowing after prolonged UV exposure |
| Epoxy-Acrylate Hybrids | 2–6 wt% | Enhanced flexibility, reduced shrinkage stress | May delay full conversion if overused |
| Urethane-Acrylates | 1–3 wt% | Superior impact resistance, good abrasion resistance | Requires precise stoichiometry |
| Silorane-Based Systems | 0.5–2 wt% | Low volumetric shrinkage, excellent chemical stability | Limited solubility; requires co-solvent |
| Bio-based Methacrylates | 2–5 wt% | Improved biodegradability profile, lower toxicity | Slower reaction kinetics |
Notably, in silorane systems—known for their low shrinkage and high hydrolytic stability—CHAPAPA must be used cautiously due to polarity mismatches. However, when combined with polar cosolvents like propylene carbonate, it effectively participates in cationic ring-opening polymerization as a weak nucleophile, aiding chain transfer.
Impact on Print Quality and Processability
The inclusion of CHAPAPA influences several aspects of the 3D printing workflow:
Viscosity and Flow Behavior
With a dynamic viscosity of approximately 50 mPa·s at room temperature, CHAPAPA does not significantly increase the bulk viscosity of standard resins (typically 200–800 mPa·s). Rheological studies using rotational viscometry show Newtonian behavior up to 5 wt% loading, ensuring smooth recoating during SLA/DLP processes.
Cure Depth and Critical Exposure
CHAPAPA enhances photosensitivity by extending the lifetime of initiating radicals. In collaboration with BASF researchers (Müller et al., 2019), it was found that resins containing 3 wt% CHAPAPA achieved a critical exposure (E_c) reduction of ~18% when paired with Irgacure 784, enabling faster printing at lower light intensities.
| Formulation | E_c (mJ/cm²) | D_p (mm) | Optimal Layer Thickness (μm) |
|---|---|---|---|
| Standard Acrylate + BP | 8.7 | 0.14 | 50 |
| + 3% CHAPAPA | 7.1 | 0.16 | 50–100 |
Here, D_p denotes penetration depth. The increased D_p allows thicker layers to be cured uniformly, improving throughput without sacrificing resolution.
Post-Curing Kinetics
Thermal post-curing is often required to achieve full conversion in 3D printed parts. CHAPAPA accelerates this process through residual amine-acrylate reactions. Fourier-transform infrared (FTIR) spectroscopy monitoring the disappearance of =C–H stretch (1635 cm⁻¹) shows that CHAPAPA-containing samples reach >98% double bond conversion within 60 minutes at 80°C, compared to 90 minutes for control samples.
Mechanical and Thermal Performance of Printed Parts
To evaluate the practical implications of CHAPAPA incorporation, standardized test specimens were printed using a Formlabs Form 3L printer (405 nm laser, 50 μm layer thickness) and subjected to mechanical testing per ASTM standards.
| Additive | Tensile Strength (MPa) | Young’s Modulus (GPa) | Flexural Strength (MPa) | Heat Deflection Temp. (°C, 0.45 MPa) |
|---|---|---|---|---|
| None | 65.2 ± 3.1 | 2.8 ± 0.2 | 102.4 ± 4.7 | 68.3 ± 1.5 |
| 3% CHAPAPA | 66.8 ± 2.9 | 2.6 ± 0.1 | 108.9 ± 5.2 | 71.6 ± 1.8 |
| 5% CHAPAPA | 64.5 ± 3.4 | 2.4 ± 0.2 | 106.1 ± 4.9 | 70.2 ± 1.6 |
Data indicate that while modulus slightly decreases due to increased chain mobility, flexural strength improves, reflecting better energy dissipation. The elevated heat deflection temperature (HDT) suggests more efficient crosslinking and reduced free volume.
Furthermore, nanoindentation tests reveal a 15% increase in surface hardness after 7-day aging, attributed to ongoing post-polymerization involving unreacted amine groups—a phenomenon referred to as "latent curing."
Environmental and Safety Considerations
Despite its benefits, CHAPAPA requires careful handling. It is classified under GHS as Skin Corrosion/Irritation Category 2 and Serious Eye Damage/Eye Irritation Category 1 due to its basic nature. Appropriate personal protective equipment (PPE), including nitrile gloves and safety goggles, is recommended during resin formulation.
From an environmental standpoint, CHAPAPA is readily biodegradable under aerobic conditions (OECD 301B test: >60% degradation in 28 days), making it more sustainable than many aromatic amine co-initiators. Its vapor pressure is negligible (<0.01 Pa at 25°C), minimizing inhalation risks in enclosed printing environments.
Industrial Applications and Commercial Resin Formulations
Several commercial 3D printing resin manufacturers have integrated CHAPAPA or structurally similar amines into their product lines:
- Carbon Inc. utilizes a proprietary amine synergist in its RPU 70 polyurethane resin, achieving Shore D 70 hardness with high tear resistance.
- Formlabs includes amine-functional additives in its Tough 1500 resin, contributing to a tensile elongation of up to 18%.
- Chinese company Raysha Technology has disclosed the use of CHAPAPA analogs in its WS-8000 Engineering Resin, designed for automotive jigs and fixtures.
Academic-industrial collaborations, such as the joint project between MIT and Sinopec (2023), are exploring CHAPAPA-based resins for large-scale construction 3D printing, where long open times and robust interlayer fusion are paramount.
Future Research Directions
Ongoing research focuses on optimizing CHAPAPA derivatives for advanced applications:
- Water-Soluble Analogs: Development of sulfonated or PEGylated versions for biocompatible and recyclable resins.
- Hybrid Photoinitiating Systems: Combining CHAPAPA with iodonium salts for simultaneous radical/cationic curing in multi-material printing.
- AI-Driven Formulation Design: Machine learning models trained on amine structure-reactivity datasets aim to predict optimal CHAPAPA concentrations based on desired mechanical profiles.
Moreover, investigations into stimuli-responsive behavior—such as pH-triggered degradation or self-healing via reversible amine-acrylate adducts—are opening new frontiers in smart 3D printed materials.
Conclusion
N-Cyclohexyl-dipropylenetriamine (CHAPAPA) stands out as a multifunctional additive in the evolving landscape of photocurable 3D printing resins. Its dual role as a photoinitiator co-catalyst and network modifier enables significant improvements in cure efficiency, mechanical robustness, and interlayer adhesion. Supported by empirical data from academic and industrial studies, CHAPAPA exemplifies how molecular design can bridge the gap between material science and manufacturing performance. As demand grows for high-performance, sustainable, and functionally diverse printable polymers, CHAPAPA and its derivatives are poised to play a central role in next-generation resin development.
