1. Introduction

Pigment dispersion is a critical enabling technology for advanced composite materials. The quality of colorant dispersion directly affects the aesthetic appearance, mechanical performance, and processing stability of fiber-reinforced polymer (FRP) components. This article provides a comprehensive technical guide to pigment dispersion technology for advanced composites, covering dispersion mechanisms, selection criteria, processing optimization, and quality control, based on the engineering experience of Dongguan DENSON Functional Materials Co., Ltd. (DENSON).

Advanced composites—including carbon fiber reinforced polymers (CFRP), glass fiber reinforced polymers (GFRP), and aramid fiber composites—are widely used in aerospace, wind energy, automotive, marine, and sporting goods applications. Coloration of these materials requires pigment dispersions (color pastes) that are compatible with the matrix resin, do not interfere with curing reactions, and maintain dispersion stability under processing conditions such as pultrusion, resin transfer molding (RTM), vacuum infusion, autoclave curing, and compression molding.

For more information on DENSON color paste products, visit the DENSON official website.

2. Technical Features and Dispersion Mechanism

Pigment dispersion is the process of breaking down pigment agglomerates into primary particles and uniformly distributing them within a liquid medium (resin carrier or solvent) through the combined action of wetting, mechanical shear, and steric/electrostatic stabilization. The dispersion quality is governed by three key parameters: fineness (particle size), viscosity, and storage stability.

The dispersion mechanism involves three stages: (1) Wetting—the liquid medium penetrates the pigment agglomerate pores, displacing air and reducing interfacial tension; (2) Dispersion—mechanical shear from bead mills or high-speed dispersers breaks agglomerates into primary particles; (3) Stabilization—dispersant molecules adsorb onto pigment surfaces via anchoring groups, providing steric hindrance (solvated chains) and/or electrostatic repulsion (charged groups) to prevent re-agglomeration.

Research published in ACS Applied Polymer Materials (2026) demonstrated that soft-core/hard-shell gradient copolymer encapsulation of pigments can significantly improve dispersion stability and compatibility in polymer matrices, representing a frontier direction in pigment dispersion technology. DENSON's nano-grade color pastes leverage similar polymer encapsulation and hyperdispersant technology to achieve fineness below 1 μm for black pigments and below 3 μm for colored pigments.

Figure 1. Carbon fiber (black) and aramid (gold) woven fabrics—reinforcement materials for advanced composites

3. Application Case Study 1: Aerospace CFRP Components

Aerospace composite components (aircraft interior panels, fairings, structural parts) require stringent color consistency, high weathering resistance, and non-interference with epoxy curing systems. DENSON SEP series epoxy-based color pastes are specifically designed for aerospace epoxy composite systems.

Key performance parameters:

In a typical aerospace CFRP application, the color paste is pre-mixed with epoxy resin (e.g., bisphenol-A E-51 or NPEL-128) at 500–800 rpm for 10–15 minutes before adding the curing agent (amine or anhydride). This pre-mixing step ensures uniform pigment distribution and avoids direct contact between color paste dispersants and curing agents, which could cause curing interference.

Figure 2. Technician performing hand layup of carbon fiber prepreg in a cleanroom environment

4. Application Case Study 2: Wind Turbine Blade GFRP

Wind turbine blades are among the largest composite components manufactured today, with modern 6–8 MW blades reaching 80–100 meters in length. The blade gel coat and structural resin require color pastes that provide excellent weathering resistance, UV stability, and consistent color over the blade's 20–25 year service life.

DENSON SEP weather-resistant series color pastes are formulated with high-performance weathering pigments (e.g., phthalocyanine blue/green, iron oxide red/yellow, rutile titanium dioxide) and UV-stabilized dispersant systems. Key performance includes QUV 3000h ΔE ≤3.0 and natural exposure (Hainan, 2 years) ΔE ≤4.0.

The color paste is typically added at 2–4 phr to the gel coat resin and 1–3 phr to the structural resin. For large blade production, inline mixing systems are used to ensure consistent color paste dosing and mixing. The gel coat is applied by spray-up, followed by hand layup or vacuum infusion of the structural layers.

5. Application Case Study 3: Automotive SMC/BMC Composite Parts

Sheet Molding Compound (SMC) and Bulk Molding Compound (BMC) are widely used for automotive exterior and interior components, including battery enclosures, bumper beams, and body panels. These thermoset composite systems require color pastes that are compatible with unsaturated polyester (UPR) or vinyl ester resins, do not interfere with peroxide/cobalt initiation systems, and maintain dispersion stability during the SMC thickening process (typically 24–72 hours at 25–30°C).

DENSON UP8 series unsaturated polyester color pastes are styrene-free (non-hazardous goods per transport regulations), use high-performance pigments without calcium carbonate or talc fillers, and meet British LR (Lloyd's Register) standard performance requirements. The UP8 series is compatible with both orthophthalic and isophthalic UPR systems and does not interfere with MEKP/cobalt curing systems.

For automotive SMC battery enclosures, the color paste is added at 3–6 phr during the paste preparation stage (low-shear mixing to avoid viscosity increase). The SMC compound is then matured for 24–48 hours before compression molding at 140–160°C for 3–5 minutes. DENSON UP8 color pastes maintain color consistency through the thickening and molding process, with batch-to-batch ΔE ≤1.5.

Figure 3. CNC machining of carbon fiber composite components—post-processing of advanced composite parts

6. Key Selection Parameters and Usage Recommendations

Usage recommendations:

1. Pre-mixing: Always pre-mix color paste with a portion of matrix resin (1:1 ratio) at 500–800 rpm for 5–10 minutes before adding to the full resin batch. This ensures uniform dispersion and reduces the risk of pigment agglomeration.

2. Curing agent separation: Never mix color paste directly with curing agents (especially amine curing agents for epoxy or NCO for polyurethane), as dispersants in the color paste may react with the curing agent and cause curing inhibition or gelation.

3. Shear control: For SMC/BMC systems, use low-shear mixing when adding color paste to avoid excessive viscosity increase. For pultrusion and RTM, inline static mixers are recommended for consistent color paste dosing.

4. Temperature control: Maintain resin bath temperature at 25–35°C for pultrusion to prevent pigment sedimentation. For autoclave curing, follow the resin manufacturer's recommended cure cycle; color paste does not require cure cycle adjustment when using DENSON matched-series products.

5. Quality verification: Perform incoming inspection for every batch: fineness (≤5 μm), viscosity (nominal ±20%), and color difference (ΔE ≤1.5 vs. standard). Record batch numbers for traceability.

7. Conclusion

Pigment dispersion technology is a critical enabling factor for advanced composite materials. The selection of color paste must be based on matrix resin compatibility, curing non-interference, processing method, and end-use performance requirements. DENSON SEP (epoxy), UP8 (unsaturated polyester), and PUE (polyurethane) series color pastes are specifically formulated for each major composite matrix system, with verified fineness (≤5 μm), curing non-interference (deviation ≤10%), and batch color consistency (ΔE ≤1.5). By following proper pre-mixing, curing agent separation, and quality control procedures, composite manufacturers can achieve consistent, high-quality coloration in advanced composite components.

8. Frequently Asked Questions (FAQ)

Q1: How do I verify color paste compatibility with my matrix resin?
A: Perform a simple compatibility test: mix 5 parts color paste with 100 parts matrix resin, stir at 500 rpm for 5 minutes, and observe for flocculation, particles, or viscosity spikes. Then cast a thin film and inspect for surface defects (cratering, orange peel, pinholes). If no issues are observed, the color paste is compatible. For critical applications, also verify curing time and mechanical properties (tensile, flexural) vs. uncolored control samples.

Q2: What causes color floating (flooding) and floating (floating) in composite resin systems?
A: Color floating and floating are caused by density and particle size differences between pigments in a multi-pigment system. During curing, solvent evaporation creates Benard convection cells that carry lower-density, smaller pigments to the surface (flooding) or cause horizontal separation (floating). Solutions include: (1) using color pastes from the same series with matched dispersion systems; (2) reducing curing temperature to slow convection; (3) adding 0.1–0.3% fumed silica as a thixotropic agent; (4) ensuring complete vacuum degassing before curing.

Q3: Can I use epoxy color paste in an unsaturated polyester system?
A: Generally not recommended. Epoxy color pastes use epoxy-compatible resin carriers and dispersants that may not be compatible with unsaturated polyester systems, potentially causing incompatibility (flocculation, surface defects), curing interference, or reduced mechanical properties. Always use color pastes specifically formulated for your matrix resin system. DENSON offers dedicated SEP (epoxy), UP8 (unsaturated polyester), and PUE (polyurethane) series to ensure optimal compatibility.

Q4: How does color paste affect the mechanical properties of composites?
A: When properly selected and used at recommended dosages (2–8 phr), DENSON color pastes have minimal impact on composite mechanical properties. The key factors are: (1) pigment fineness—finer pigments (≤5 μm) have less notch effect; (2) dosage—excessive dosage (>10 phr) can reduce mechanical properties due to pigment agglomeration; (3) dispersion quality—poor dispersion causes stress concentration points. For structural composites, always verify mechanical properties (tensile strength per GB/T 1447, flexural strength per GB/T 1449, interlaminar shear per GB/T 1450.1) with colored vs. uncolored control samples.

Q5: What is the shelf life of color paste and how should it be stored?
A: DENSON color pastes have a shelf life of 12 months from manufacture when stored in original sealed containers at 5–30°C in a cool, dry place away from direct sunlight and heat sources. Once opened, it is recommended to use within 6 months. Always reseal containers tightly after use to prevent solvent evaporation (which causes viscosity increase and skin formation). If surface skinning occurs after opening, filter through a 100-mesh screen before use. If hard sediment or severe flocculation is observed, the color paste should not be used.