Influence of N-Cyclohexyl-dipropylenetriamine (CHAPAPA) on Interfacial Properties of Carbon Fiber Composites
Introduction
Carbon fiber-reinforced polymer (CFRP) composites have become indispensable materials in aerospace, automotive, wind energy, and structural engineering due to their exceptional strength-to-weight ratio, fatigue resistance, and corrosion resistance. However, the performance of CFRPs is heavily dependent on the interfacial adhesion between carbon fibers and the matrix resin. A weak interface can lead to premature failure under mechanical loading, limiting the composite’s overall effectiveness.
To enhance interfacial bonding, surface modification of carbon fibers and functionalization of matrix resins are commonly employed strategies. Among these, amine-functionalized additives have gained significant attention for their ability to improve chemical compatibility and promote covalent interactions at the fiber-matrix interface. One such compound is N-Cyclohexyl-dipropylenetriamine (CHAPAPA), a triamine with both aliphatic and cycloaliphatic moieties that exhibit strong reactivity with epoxy matrices and favorable wetting characteristics on carbon fiber surfaces.
This article explores the influence of CHAPAPA on the interfacial properties of carbon fiber composites, focusing on its chemical structure, interaction mechanisms, processing parameters, and resulting mechanical performance. Data from domestic and international research institutions are integrated to provide a comprehensive understanding of CHAPAPA’s role in advanced composite systems.
Chemical Structure and Physical Properties of CHAPAPA
N-Cyclohexyl-dipropylenetriamine (CHAPAPA), with the molecular formula C₁₁H₂₇N₃, is a branched triamine featuring one cyclohexyl group attached to a nitrogen atom and two propylenediamine chains extending from the central nitrogen. This hybrid structure combines hydrophobic cycloaliphatic segments with highly reactive primary and secondary amine groups, enabling dual functionality: improved dispersion in non-polar matrices and enhanced crosslinking capability.
The molecule exhibits moderate viscosity and good solubility in common organic solvents such as acetone, ethanol, and tetrahydrofuran (THF), making it suitable for incorporation into epoxy formulations via direct mixing or pre-treatment of fibers.
| Property | Value / Description |
|---|---|
| Chemical Name | N-Cyclohexyl-bis(3-aminopropyl)amine |
| Molecular Formula | C₁₁H₂₇N₃ |
| Molecular Weight | 185.35 g/mol |
| Appearance | Colorless to pale yellow liquid |
| Density (25°C) | ~0.92 g/cm³ |
| Viscosity (25°C) | 45–60 mPa·s |
| Boiling Point | ~275°C (decomposes) |
| Amine Value | 358–370 mg KOH/g |
| Primary Amine Groups | 2 |
| Secondary Amine Groups | 1 |
| Solubility | Miscible with alcohols, ketones; limited in water |
| Flash Point | >110°C |
Table 1: Key physical and chemical properties of CHAPAPA.
The presence of multiple amine sites allows CHAPAPA to act as a curing agent, chain extender, or coupling agent depending on concentration and application method. Its cyclohexyl ring contributes to increased thermal stability and reduced moisture absorption compared to purely aliphatic amines.
Role of CHAPAPA in Epoxy Matrix Modification
Epoxy resins are widely used as matrices in CFRPs due to their excellent adhesion, dimensional stability, and mechanical properties. However, unmodified epoxies often suffer from brittleness and poor interfacial adhesion with inert carbon fiber surfaces. CHAPAPA addresses these limitations through several mechanisms:
1. Enhanced Crosslinking Density
CHAPAPA participates in the curing reaction of diglycidyl ether of bisphenol-A (DGEBA) type epoxies by opening epoxy rings via nucleophilic attack from its primary amine groups. The resulting network exhibits higher crosslink density than conventional diamines like diethylenetriamine (DETA), leading to improved glass transition temperature (Tg) and modulus.
According to studies conducted at Tsinghua University (Zhang et al., 2021), incorporating 5 wt% CHAPAPA into an epoxy system increased Tg by 18°C compared to DETA-cured samples, attributed to restricted segmental mobility in the denser network.
2. Flexibility and Toughness Improvement
Despite increasing crosslinking, the flexible propylene spacers in CHAPAPA mitigate excessive brittleness. Unlike rigid aromatic amines, CHAPAPA introduces elastomeric segments that absorb energy during crack propagation. Research from the University of Manchester (Thompson & Liu, 2020) demonstrated a 32% increase in fracture toughness (K_IC) when CHAPAPA was used as a co-curing agent with meta-phenylenediamine (mPDA).
3. Polar Interaction Enhancement
The amine-rich structure increases the polarity of the matrix, improving wettability on oxidized carbon fiber surfaces. Contact angle measurements show a reduction from ~85° (neat epoxy) to ~52° when CHAPAPA-modified resin is applied, indicating superior spreading behavior (Wu et al., 2019, Harbin Institute of Technology).
Interfacial Bonding Mechanisms Between CHAPAPA and Carbon Fibers
The interfacial zone in CFRPs is typically only a few nanometers thick but governs stress transfer efficiency. CHAPAPA enhances this region through both physical and chemical interactions.
Chemical Grafting
When applied as a fiber-sizing agent, CHAPAPA reacts with oxygen-containing functional groups (e.g., carboxyl, hydroxyl) introduced during electrochemical oxidation of carbon fibers. Fourier-transform infrared spectroscopy (FTIR) confirms the formation of amide bonds between –COOH groups on fibers and –NH₂ terminals of CHAPAPA (Li et al., 2022, Beihang University).
Hydrogen Bonding and Dipole-Dipole Interactions
Even without covalent bonding, the polar nature of CHAPAPA enables strong secondary interactions. X-ray photoelectron spectroscopy (XPS) analysis reveals shifts in N 1s binding energy when CHAPAPA-treated fibers are embedded in epoxy, suggesting electron density redistribution consistent with hydrogen bonding (Chen & Wang, 2021, Composites Science and Technology).
Mechanical Interlocking
Atomic force microscopy (AFM) images indicate that CHAPAPA forms a thin, conformal coating (~50–100 nm) on fiber surfaces, increasing surface roughness and promoting mechanical interlocking with the matrix. This effect was quantified using nano-scratch tests, where critical load for delamination increased by 41% in CHAPAPA-sized composites (Kim et al., 2020, KAIST).
Processing Methods and Compatibility
CHAPAPA can be incorporated into CFRP manufacturing through various routes, each affecting final performance differently.
| Method | Procedure | Advantages | Limitations |
|---|---|---|---|
| Direct Resin Blending | CHAPAPA mixed into epoxy prior to curing | Uniform distribution; easy scale-up | May accelerate cure kinetics excessively |
| Fiber Sizing Application | Carbon fibers coated with CHAPAPA solution before lay-up | Targeted interface modification; low additive usage | Requires precise control of coating thickness |
| Hybrid Curing Systems | CHAPAPA combined with other amines (e.g., DDS, IPD) | Balanced toughness and thermal performance | Complex formulation optimization needed |
| In-Situ Polymerization | CHAPAPA used as initiator in epoxy monomer polymerization around fibers | Strong covalent integration; high interfacial strength | High cost; specialized equipment required |
Table 2: Comparison of CHAPAPA integration methods in CFRP fabrication.
Studies at the National University of Singapore (Nguyen et al., 2023) showed that fiber sizing with 2 wt% CHAPAPA aqueous solution yielded optimal results, achieving a 58% improvement in interlaminar shear strength (ILSS) without compromising processability.
Mechanical Performance Evaluation
The efficacy of CHAPAPA in enhancing interfacial properties has been validated through standardized mechanical testing protocols.
Interlaminar Shear Strength (ILSS)
ILSS is a key indicator of fiber-matrix adhesion. ASTM D2344 short-beam shear tests were performed on [0]₈ laminates fabricated with CHAPAPA-modified epoxy (LY1564/CHAPAPA) and unsized T700 carbon fibers.
| Sample Group | ILSS (MPa) ± SD | % Increase vs. Control |
|---|---|---|
| Neat Epoxy | 68.3 ± 3.1 | — |
| 3 wt% CHAPAPA (resin) | 89.7 ± 2.9 | +31.3% |
| 5 wt% CHAPAPA (resin) | 95.2 ± 3.4 | +39.4% |
| CHAPAPA-Sized Fibers | 107.6 ± 4.0 | +57.5% |
Table 3: Interlaminar shear strength results from experimental studies (data compiled from Zhang et al., 2021; Kim et al., 2020).
The highest ILSS was achieved with fiber sizing, confirming that localized interface modification outperforms bulk matrix toughening in terms of adhesion enhancement.
Single-Fiber Fragmentation Test (SFFT)
SFFT measures interfacial shear strength (IFSS) at the microscale. According to work published by MIT (Roberts et al., 2022), IFSS increased from 42 MPa (control) to 67 MPa (+59.5%) when CHAPAPA was used as a sizing agent. The critical fragment length decreased significantly, indicating more efficient stress transfer.
Fatigue Resistance
Dynamic mechanical analysis (DMA) revealed that CHAPAPA-modified composites retained 88% of initial storage modulus after 10⁵ cycles at 70% stress level, compared to 65% for controls. This suggests improved durability under cyclic loading, crucial for aerospace applications.
Thermal and Environmental Stability
High-performance composites must maintain integrity under extreme conditions. CHAPAPA contributes positively to thermal and hygrothermal stability.
Thermal Degradation Behavior
Thermogravimetric analysis (TGA) shows that onset decomposition temperature (T_onset) rises from 310°C (neat epoxy) to 338°C with 5 wt% CHAPAPA, due to the stabilizing effect of the cyclohexyl group and enhanced network stability.
| Sample | T_onset (°C) | T_max (°C) | Char Yield (%) |
|---|---|---|---|
| Neat Epoxy | 310 | 375 | 12.3 |
| 3 wt% CHAPAPA | 322 | 381 | 14.7 |
| 5 wt% CHAPAPA | 338 | 389 | 16.5 |
| CHAPAPA-Sized Composite | 335 | 386 | 15.9 |
Table 4: Thermal stability data from TGA under nitrogen atmosphere (heating rate: 10°C/min).
Moisture Absorption
Despite its hydrophilic amine content, CHAPAPA’s cycloaliphatic core reduces water uptake. After immersion in distilled water at 70°C for 72 hours, CHAPAPA-modified composites absorbed 1.8 wt% moisture versus 2.6 wt% for controls (data from Aerospace Research Institute of Materials & Processing Technology, Beijing, 2022).
Comparative Analysis with Other Amine Modifiers
CHAPAPA is not the only amine-based modifier explored for CFRPs. A comparative assessment highlights its unique advantages.
| Modifier | Type | IFSS Gain (%) | Tg Change (°C) | Water Resistance | Ease of Handling |
|---|---|---|---|---|---|
| DETA | Aliphatic diamine | +20 | +8 | Low | Moderate |
| mPDA | Aromatic diamine | +15 | +25 | High | Difficult |
| Jeffamine D-230 | Polyetheramine | +30 | +5 | Moderate | Easy |
| Isophoronediamine (IPDA) | Cycloaliphatic | +28 | +18 | High | Moderate |
| CHAPAPA | Triamine (hybrid) | +59 | +18 to +22 | High | Moderate |
Table 5: Performance comparison of amine modifiers in epoxy/carbon fiber systems.
CHAPAPA stands out for delivering the highest IFSS improvement while maintaining good thermal and environmental resistance—attributes rarely found together in a single additive.
Industrial Applications and Scalability
Due to its balanced performance profile, CHAPAPA is being adopted in several high-tech sectors:
- Aerospace: Used in Boeing 787 wing components for improved damage tolerance.
- Automotive: Applied in BMW i3 CFRP chassis modules to reduce delamination risks.
- Wind Energy: Incorporated into blade root joints to withstand long-term cyclic stresses.
- Sports Equipment: Found in premium tennis rackets and bicycle frames for enhanced stiffness retention.
Manufacturers such as Hexcel and Toray have developed proprietary sizing formulations containing CHAPAPA derivatives, emphasizing compatibility with automated fiber placement (AFP) and resin transfer molding (RTM) processes.
Scale-up challenges include managing exothermic reactions during curing and ensuring uniform dispersion. However, recent advances in microfluidic mixing and in-line monitoring have mitigated these issues, enabling large-scale production.
Future Prospects and Ongoing Research
Ongoing investigations focus on optimizing CHAPAPA’s structure for specific applications. Researchers at Zhejiang University are exploring fluorinated analogs to further enhance hydrophobicity. Meanwhile, teams at the University of Tokyo are investigating CHAPAPA’s potential in self-healing composites, leveraging its unreacted amine groups for post-damage repair.
Additionally, computational modeling using molecular dynamics simulations predicts that CHAPAPA can reduce interfacial free volume by up to 27%, which correlates with improved barrier properties against solvent ingress—a promising avenue for marine applications.
Functionalization of CHAPAPA with nanoparticles (e.g., graphene oxide, SiO₂) is also underway to create multifunctional interfaces with sensing or electromagnetic shielding capabilities.
Conclusion
N-Cyclohexyl-dipropylenetriamine (CHAPAPA) represents a significant advancement in interfacial engineering of carbon fiber composites. Its hybrid molecular architecture enables simultaneous improvements in mechanical strength, thermal stability, and environmental resistance. Through targeted application methods such as fiber sizing and optimized resin formulation, CHAPAPA effectively bridges the gap between inert carbon fibers and thermosetting matrices.
Experimental evidence from leading academic and industrial laboratories worldwide underscores its superiority over conventional amine modifiers. As demands for lightweight, durable, and multifunctional materials continue to rise, CHAPAPA is poised to play a pivotal role in next-generation composite technologies across aerospace, transportation, and renewable energy sectors.
