1. Introduction

Fiber-reinforced polymer (FRP) composites have become essential materials across wind energy, aerospace, automotive, and construction industries due to their high strength-to-weight ratio, corrosion resistance, and design flexibility. Coloration is a critical aspect of composite manufacturing, affecting both aesthetic quality and brand identity. Color pastes (pigment dispersions) are the preferred coloring method for composites because they offer easy handling, uniform color distribution, and precise color matching. Dongguan DENSON Functional Materials Co., Ltd. has developed specialized composite colorant systems that address the unique challenges of pigment dispersion and interface compatibility in resin matrices. This article examines the dispersion mechanism of DENSON composite colorants and their interface compatibility with different resin systems, supported by three application case studies.

2. Technical Features and Mechanism

Composite colorant dispersion refers to the process of breaking down pigment agglomerates into primary particles and uniformly distributing them within the resin matrix while maintaining long-term stability. The key technical challenges in composite coloring include: (1) fiber-resin interface regions that repel pigment particles; (2) resin flow during molding that causes pigment migration; and (3) curing exotherm that demands high pigment thermal stability.

DENSON composite colorants employ a three-pronged technical approach:

(1) Pigment surface modification: Silane coupling agents are used to modify inorganic pigment surfaces, improving interfacial bonding with resin matrices and reducing pigment accumulation at fiber-resin interfaces.

(2) Low-viscosity carrier system: Carriers with viscosity ≤500 mPa·s at 25°C ensure rapid and uniform dispersion in resins without compromising fiber impregnation.

(3) Heat-resistant pigment selection: Pigments rated for ≥250°C service temperature withstand epoxy and phenolic resin curing at 150-200°C, with post-cure color difference ΔE ≤1.0.

The dispersion mechanism involves wetting (pigment surface covered by resin carrier), deagglomeration (mechanical shear breaking agglomerates), and stabilization (steric and electrostatic barriers preventing re-agglomeration). DENSON colorants achieve particle size D90 ≤5μm (ISO 1524:2020), ensuring smooth composite surfaces.

3. Application Case Study 1: GFRP Wind Turbine Blade Coloring

A European wind turbine manufacturer used vacuum-assisted resin transfer molding (VARTM) to produce 60-meter glass fiber reinforced polymer (GFRP) blades, requiring uniform gel coat color and UV resistance for 20-year outdoor service. DENSON unsaturated polyester-based colorant was selected at 4% loading for the gel coat and 2.5% for the structural resin.

Key performance results: blade surface color difference ΔE ≤0.8, QUV 3000h aging ΔE ≤2.5 (ISO 4892-3), gel coat thickness 0.5-0.6mm with no pinholes, interlaminar shear strength ≥45MPa (ISO 14130), and flexural strength ≥550MPa (ISO 14125). The colorant demonstrated excellent dispersion in the low-viscosity VARTM resin system, with no pigment filtration during resin injection.

4. Application Case Study 2: CFRP Aerospace Component Coloring

An aerospace composite supplier produced carbon fiber reinforced polymer (CFRP) interior panels using autoclave curing at 180°C, requiring precise color matching, low volatile organic compound (VOC) emissions, and compliance with aerospace flame retardancy standards. DENSON epoxy-based colorant with high-pigment-load formulation (35% pigment content) was used at 3.5% loading.

Key performance results: panel color difference ΔE ≤0.5, 60° gloss ≥85GU, heat resistance at 180°C/2h with no color shift, OSU heat release ≤65/65 (FAR 25.853), smoke density Ds ≤100 (ASTM E662), and CFRP flexural strength ≥1200MPa (ASTM D790). The colorant's surface-modified pigments prevented fiber-resin interface debonding, maintaining structural integrity under autoclave curing conditions.

5. Application Case Study 3: SMC Automotive Part Coloring

A Tier-1 automotive supplier manufactured sheet molding compound (SMC) body panels using compression molding at 150°C, requiring Class A surface finish, excellent weatherability, and consistent color across high-volume production. DENSON SMC-specific colorant with low-styrene carrier and surface-treated pigments was applied at 3% loading.

Key performance results: panel color difference ΔE ≤1.0 (batch-to-batch), 20° gloss ≥90GU, SMC thickening viscosity 300,000-800,000 mPa·s with no pigment settling, 50°C/30-day storage stability with no hard settlement, xenon arc 2000h aging ΔE ≤2.0 (SAE J2527), and coating adhesion 0B (ASTM D3359). The colorant's anti-migration formulation prevented pigment bleed during high-temperature compression molding.

6. Key Selection Parameters and Usage Recommendations

| Composite Type | Recommended Colorant | Pigment Content | Loading | Critical Parameter |

|---|---|---|---|---|

| GFRP (VARTM) | Unsaturated polyester-based | 30-40% | Gel coat 4-6% / resin 2-3% | Low viscosity ≤1000 mPa·s |

| CFRP (Autoclave) | Epoxy-based | 25-35% | 3-4% | Heat resistance ≥200°C |

| SMC (Compression) | Low-styrene SMC-specific | 35-45% | 2.5-4% | Anti-migration, storage stability |

Usage recommendations:

(1) Always conduct compatibility testing with the specific resin and curing system before full production.

(2) For VARTM processes, maintain resin viscosity ≤500 mPa·s after colorant addition to ensure proper fiber wet-out.

(3) For autoclave curing, verify colorant thermal stability at the maximum cure temperature.

(4) For SMC production, perform 50°C/7-day storage stability tests to confirm no pigment settlement.

(5) Refer to DENSON composite colorant product pages for detailed technical data sheets.

7. Conclusion

The three case studies demonstrate that DENSON composite colorants achieve reliable dispersion and interface compatibility across GFRP wind turbine blades, CFRP aerospace components, and SMC automotive parts. The key success factors are pigment surface modification, low-viscosity carrier systems, and heat-resistant pigment selection. Composite manufacturers should select colorants matched to their specific resin system and molding process, and conduct thorough small-scale and pilot-scale trials before full production. Proper colorant selection ensures uniform color, maintains mechanical properties, and meets industry-specific performance standards.

8. FAQ

Q1: What causes color variation in composite parts and how can it be prevented?

A: Color variation in composites typically results from: (1) uneven pigment dispersion due to insufficient mixing; (2) resin flow during molding causing pigment migration; (3) different curing temperatures affecting pigment stability; and (4) batch-to-batch colorant variation. Prevention measures include using high-dispersibility colorants (D90 ≤5μm), maintaining consistent mixing parameters (500-1500rpm for 10-15min), controlling mold temperature within ±5°C, and implementing incoming colorant quality control with ΔE ≤0.5 acceptance criteria. Regular color standard calibration using a spectrophotometer (CIE L*a*b* system) is recommended.

Q2: How does colorant affect the mechanical properties of composites?

A: Colorants can affect composite mechanical properties through several mechanisms: (1) carrier resin dilution of the matrix, potentially reducing flexural strength by 5-10% at >5% loading; (2) pigment particles acting as stress concentrators if dispersion is poor; (3) interface weakening if pigment accumulates at fiber-resin boundaries; and (4) curing interference from certain organic pigments (carbon black, phthalocyanine) that can retard gel time by 10-30%. To minimize impact, keep colorant loading ≤5%, use surface-treated pigments, select colorants with curing compatibility certification, and verify mechanical property retention ≥90% (ISO 14125 flexural strength) through testing.

Q3: What is the difference between color paste and color masterbatch for composites?

A: Color paste (liquid pigment dispersion) and color masterbatch (solid pigment concentrate) differ in carrier form and application method. Color paste uses liquid resin carrier (viscosity 500-5000 mPa·s), is suitable for liquid molding processes (VARTM, RTM, hand lay-up), offers easy dosing and rapid dispersion, but has shorter shelf life (6-12 months). Color masterbatch uses solid thermoplastic carrier, is suitable for thermoplastic composites and injection molding, offers convenient storage and handling, but requires higher processing temperatures for dispersion. For thermoset composites, color paste is generally preferred due to better compatibility with liquid resin systems and more uniform color distribution.

Q4: How to choose between inorganic and organic pigments for composite colorants?

A: Inorganic pigments (iron oxides, titanium dioxide, ultramarine) offer excellent heat resistance (≥300°C), weatherability, and chemical resistance, but have limited color range and lower tinting strength. Organic pigments (phthalocyanine, quinacridone, azo) offer bright, vibrant colors with high tinting strength, but generally have lower heat resistance (150-250°C) and weatherability. For composites: (1) outdoor applications (wind blades, construction) - prefer inorganic pigments for long-term durability; (2) high-temperature curing (autoclave ≥180°C) - require heat-resistant organic or inorganic pigments; (3) interior/consumer applications - organic pigments acceptable for color vibrancy. DENSON colorants use application-specific pigment blends to balance performance and color requirements.

Q5: What storage conditions are required for composite colorants?

A: Composite colorants should be stored at 5-35°C in sealed containers, away from direct sunlight and heat sources. Unsaturated polyester-based colorants have 12-month shelf life, epoxy-based 12 months, and SMC-specific 6 months (due to low-styrene carrier thickening). Storage above 40°C can cause resin carrier polymerization, viscosity increase, and dispersion stability loss. Storage below 5°C may cause thickening and pigment settlement, requiring warming to 25°C and re-stirring before use. Containers should be tightly sealed after each use to prevent solvent evaporation and skin formation. Expired colorants should be tested for fineness (ISO 1524), viscosity, and tinting strength before use. DENSON recommends first-in-first-out inventory management.