1. Introduction

Color pastes (pigment dispersions) play a critical role in composite materials manufacturing, providing consistent coloration while maintaining the mechanical, thermal, and electrical performance of the final composite part. The dispersion stability and interface compatibility of color pastes directly affect the quality of composite products, including color uniformity, surface finish, interlaminar adhesion, and long-term weathering resistance. Dongguan DENSON Functional Materials Co., Ltd. (DENSON) has developed specialized color paste series for composite applications — including SEP epoxy-based color pastes, PUE polyether polyol carrier color pastes, and UP8 styrene-free unsaturated polyester color pastes — engineered to meet the demanding requirements of wind energy, aerospace, automotive, and industrial composite manufacturing.

This article provides a technical review of the dispersion stability and interface compatibility mechanisms of color pastes in composite materials, covering: (1) the fundamental principles of pigment dispersion and steric stabilization in resin systems; (2) interface compatibility between color paste carriers and matrix resins (epoxy, unsaturated polyester, polyurethane); (3) three application case studies — wind blade vacuum infusion, carbon fiber HP-RTM, and SMC/BMC compression molding; (4) key selection parameters and usage recommendations for composite color pastes.

Figure 1. FRP pigment paste samples showing a range of colors for composite material coloring applications

2. Technical Features and Mechanism

Dispersion stability refers to the ability of pigment particles in a color paste to remain uniformly distributed without agglomeration, sedimentation, or flocculation over time and under processing conditions. In composite materials, color pastes are typically added at 0.5–5.0 wt% to the matrix resin, and the pigment particles must remain dispersed through the resin infusion, curing, and post-curing processes. The stabilization mechanism relies on a combination of electrostatic repulsion (for aqueous systems) and steric hindrance (for solvent-borne and 100% solids systems), where hyperdispersants with anchoring groups adsorb onto pigment surfaces while solvated polymer chains extend into the resin medium, creating a repulsive barrier that prevents particle agglomeration (ACS Applied Polymer Materials, 2026).

Interface compatibility is defined as the degree to which the color paste's carrier resin, dispersant, and solvent system are miscible with the matrix resin without causing phase separation, viscosity drift, cure inhibition, or surface defects. In composite manufacturing, poor interface compatibility can lead to several critical issues: (1) resin-rich areas or pigment agglomerates at the fiber-resin interface, reducing interlaminar shear strength (ILSS); (2) cure inhibition due to dispersant-curing agent interactions, resulting in under-cured or tacky surfaces; (3) exudation or blooming of incompatible carrier resins to the part surface, causing poor paint adhesion or cosmetic defects; (4) viscosity increase that impedes resin flow during infusion or injection molding.

The key factors governing dispersion stability and interface compatibility in composite color pastes include:

(1) Pigment surface treatment — inorganic pigments (iron oxides, titanium dioxide, carbon black) require surface modification with silica, alumina, or organic coatings to improve dispersant adsorption and reduce catalytic activity on resin curing. Organic pigments (phthalocyanines, quinacridones, azo pigments) benefit from resinification or derivative treatments to enhance compatibility with polar resin systems.

(2) Hyperdispersant architecture — the anchoring group must match the pigment surface chemistry (acidic anchors for basic pigments, basic anchors for acidic pigments), while the solvated chain must be soluble in the target resin medium. For epoxy systems, dispersants with aromatic or aliphatic amine anchoring groups and polyether/polyester solvated chains are preferred. For unsaturated polyester systems, dispersants with carboxylic acid anchoring groups and polyester chains provide optimal compatibility.

(3) Carrier resin selection — the carrier resin in a color paste must be chemically compatible with the matrix resin. DENSON SEP series uses an epoxy-compatible carrier that co-reacts with the matrix during cure; PUE series uses polyether polyol carriers that participate in polyurethane cure reactions; UP8 series uses a styrene-free polyester carrier compatible with unsaturated polyester resins. The carrier resin should not introduce plasticization or reduce glass transition temperature (Tg) of the cured composite.

(4) Solvent system — for solvent-borne color pastes, the solvent must be compatible with the resin system and must fully evaporate before gelation to avoid voids or porosity. In high-pressure RTM or infusion processes, low-solvent or solvent-free color pastes are preferred to minimize volatile content and ensure consistent resin viscosity.

Figure 2. Manual resin application onto carbon fiber fabric, illustrating the resin-pigment-fiber interface in composite layup

3. Application Case Study 1: Wind Turbine Blade Vacuum Infusion

Wind turbine blades represent one of the largest applications of colored composite materials, with modern blades exceeding 100 meters in length and requiring consistent gelcoat coloration over the entire surface. In vacuum-assisted resin transfer molding (VARTM), color paste is typically added to the epoxy or unsaturated polyester resin before infusion, and the colored resin must flow uniformly through the glass fiber reinforcement without pigment filtration or color variation.

A major Chinese wind blade manufacturer producing 94-meter epoxy blades tested DENSON SEP series epoxy color paste at 2.0 wt% loading. The key performance results are summarized below:

The critical finding was that the SEP color paste's epoxy-compatible carrier did not interfere with the amine curing agent, maintaining gel time within 3% of the control. The pigment dispersion remained stable during the 4-hour infusion process, with no pigment filtration observed at the flow front. The color uniformity across the 94-meter blade was ΔE ≤ 1.2, well within the industry acceptance criterion of ΔE ≤ 2.0. For more technical articles, visit the DENSON official website and Literature section.

4. Application Case Study 2: Carbon Fiber HP-RTM Automotive Components

High-pressure resin transfer molding (HP-RTM) is increasingly used for automotive carbon fiber components, with cycle times of 5–10 minutes and injection pressures of 30–80 bar. In HP-RTM, the color paste is mixed with the epoxy resin inline just before injection, requiring excellent dispersion stability under high shear and rapid cure conditions. The color paste must not cause filter blockage at the mixing head or viscosity increase that would impede high-pressure injection.

An automotive Tier 1 supplier producing carbon fiber roof panels for a premium electric vehicle evaluated DENSON SEP series color paste in a HP-RTM process with the following parameters: resin injection pressure 50 bar, mold temperature 120°C, cure time 6 min, carbon fiber fabric (200 g/m², 8 layers). Results:

The key advantage observed was that the nano-scale pigment dispersion (D90 ≤ 1.0 μm) in the SEP color paste prevented filter blockage at the HP-RTM mixing head, extending filter life from the typical 3000 shots to over 5000 shots. The high-shear mixing in the HP-RTM dosing unit did not cause pigment re-agglomeration, demonstrating robust steric stabilization. The cured parts showed uniform color with ΔE ≤ 0.8 across 10 consecutive production shots, meeting automotive Class A surface requirements.

Figure 3. Automated carbon fiber composite production line with robotic layup and preforming equipment

5. Application Case Study 3: SMC/BMC Compression Molding

Sheet molding compound (SMC) and bulk molding compound (BMC) are widely used for automotive exterior body panels, electrical enclosures, and consumer goods. In SMC/BMC, color paste is added during the compounding process, and the colored compound must remain stable during maturation (24–72 hours at 25–40°C) without color separation or viscosity drift. The compression molding process (140–160°C, 50–100 bar, 1–5 min) subjects the color paste to high temperature and pressure, requiring thermal stability of both pigments and dispersants.

DENSON UP8 series styrene-free unsaturated polyester color paste was evaluated in an SMC formulation for automotive exterior panels. The UP8 series uses high-performance pigments without calcium carbonate or talc fillers, meets UK LR standard requirements, and is styrene-free (non-hazardous for transport). Key results:

The UP8 color paste demonstrated superior performance in SMC applications: the styrene-free formulation eliminated styrene emissions during compression molding (improving workplace air quality), the high-pigment formulation without fillers provided better color uniformity and gloss, and the compatible polyester carrier did not cause viscosity drift during SMC maturation. The absence of calcium carbonate and talc fillers also improved the mechanical properties of the molded SMC parts, with flexural modulus increasing from 11.8 to 12.5 GPa.

Figure 4. Modern composite manufacturing facility with automated gantry systems for high-volume production

6. Key Selection Parameters and Usage Recommendations

Usage recommendations: (1) Always pre-mix the color paste with a portion of the base resin before adding to the full batch — this ensures uniform dispersion and avoids local pigment concentration; (2) For infusion processes, filter the colored resin through a 100–200 μm filter before infusion to remove any agglomerates; (3) For HP-RTM, use inline mixing with a static mixer and ensure the color paste dosage pump is calibrated to ±1% accuracy; (4) For SMC/BMC, add the color paste during the high-shear mixing stage (before thickener addition) to ensure uniform distribution; (5) Always conduct a small-scale cure test (50–100 g) before full production to verify gel time, color, and surface finish; (6) Store color pastes sealed at 5–35°C and stir thoroughly before use — sedimented pigment can be re-dispersed with low-shear stirring (300–500 rpm) for 10–15 minutes.

7. Conclusion

The dispersion stability and interface compatibility of color pastes are critical factors in composite materials manufacturing, directly affecting color uniformity, mechanical performance, and production efficiency. DENSON's specialized color paste series — SEP for epoxy, UP8 for unsaturated polyester, and PUE for polyurethane — are engineered with matched carrier resins, optimized hyperdispersant architectures, and nano-scale pigment dispersions that ensure stable performance across vacuum infusion, HP-RTM, and SMC/BMC compression molding processes. Selecting the correct color paste for the resin system and process, following recommended dosage and mixing procedures, and implementing proper quality control (fineness ≤10 μm, batch color ΔE ≤1.5, cure time deviation ≤10%) will ensure consistent, high-quality colored composite parts.

8. Frequently Asked Questions (FAQ)

Q1: How do I determine the correct color paste dosage for my composite resin system?

Start with the manufacturer's recommended dosage range (typically 0.5–3.0 wt% for epoxy and PU, 1.0–5.0 wt% for UPR) and prepare stepwise samples at 80%, 100%, and 120% of the midpoint. Cast 3 mm plaques under your standard cure conditions and measure color (CIE L*a*b*), gloss, and mechanical properties. The optimal dosage is the minimum loading that achieves the target color without causing cure interference or mechanical property reduction. For dark colors (black, dark blue), 0.5–1.5 wt% is usually sufficient; for light colors (white, pastels), 2.0–5.0 wt% may be required.

Q2: What causes color variation (floating or flooding) in composite parts and how can it be prevented?

Color floating/flooding in composites is typically caused by: (1) density differences between pigment particles leading to segregation during resin gelation; (2) incompatibility between color paste dispersant and matrix resin causing flocculation; (3) uneven resin flow during infusion or molding causing pigment filtration. Prevention measures: use color pastes with matched carrier resins for your system, ensure thorough low-shear mixing (300–500 rpm for 5–10 min), avoid mixing color pastes from different manufacturers without compatibility testing, filter the colored resin before infusion, and for SMC ensure the color paste is added during high-shear compounding rather than post-added.

Q3: Can color pastes affect the curing reaction of composite resins?

Yes, some color pastes can interfere with curing. Carbon black can adsorb amine curing agents in epoxy systems, extending gel time. Certain organic pigments (e.g., phthalocyanine blue) can catalyze or inhibit peroxide cure in unsaturated polyester. Dispersants with acidic or basic functional groups can react with curing agents. To prevent cure interference: select color pastes specifically formulated for your resin system (such as DENSON SEP for epoxy, UP8 for UPR), always conduct a gel time comparison test with and without color paste, and if cure inhibition occurs, increase curing agent dosage by 5–10% or reduce color paste loading. Avoid using universal color pastes not validated for composite resin systems.

Q4: How should I store and handle color pastes to maintain dispersion stability?

Store color pastes in their original sealed containers at 5–35°C (5–30°C for UPR-based pastes), away from direct sunlight and heat sources. Before use, stir the entire container thoroughly with a low-shear mixer (300–500 rpm) for 10–15 minutes, ensuring the mixing blade reaches the bottom where sediment may have accumulated. If hard sediment is present (cannot be re-dispersed by stirring), the paste should be discarded. Once opened, use within 3 months and keep the container tightly sealed between uses to prevent solvent evaporation and skin formation. Do not return unused color paste to the original container to avoid contamination.

Q5: What is the difference between styrene-free and conventional unsaturated polyester color pastes?

Conventional UPR color pastes use styrene as a reactive diluent, which provides good compatibility but emits volatile organic compounds (VOCs) during molding and is classified as a hazardous material for transport. Styrene-free color pastes (like DENSON UP8 series) use alternative reactive diluents or are 100% solids, eliminating styrene emissions and reducing workplace exposure. They typically use higher-performance pigments without fillers (calcium carbonate, talc), providing better color strength, gloss, and mechanical properties. Styrene-free pastes may have slightly higher viscosity and require adjusted processing parameters, but offer significant environmental, health, and safety (EHS) benefits, especially in closed-mold processes where ventilation is limited.