1. Introduction

Selecting the right colorant and optimizing the coloring process are critical for composite manufacturers seeking consistent color quality, production efficiency, and cost control. Different composite manufacturing processes—hand lay-up, resin transfer molding (RTM), pultrusion, filament winding, and sheet molding compound (SMC)—impose distinct requirements on colorant viscosity, pigment particle size, curing compatibility, and storage stability. Dongguan DENSON Functional Materials Co., Ltd. provides technical guidance on colorant selection parameters and process optimization for major composite manufacturing methods. This article covers key selection criteria, three process optimization case studies (hand lay-up, RTM, and pultrusion), and practical recommendations for achieving consistent color quality in composite production.

2. Technical Features and Mechanism

Composite colorant selection parameters refer to the measurable properties that determine a colorant's suitability for a specific manufacturing process and end-use application. The primary selection parameters include:

(1) Viscosity: Critical for process compatibility. Hand lay-up requires 500-2000 mPa·s, RTM requires ≤500 mPa·s, pultrusion requires 1000-5000 mPa·s, and SMC requires 5000-20000 mPa·s (Brookfield, 25°C).

(2) Pigment particle size: Measured by Hegman gauge (ISO 1524:2020). Fineness ≥7 grade (D90 ≤5μm) is required for smooth surfaces and Class A finishes.

(3) Pigment content: Typically 30-45%. Higher content reduces required loading, minimizing carrier dilution effects on mechanical properties.

(4) Thermal stability: Must exceed maximum cure temperature by ≥30°C. For autoclave curing at 180°C, colorant thermal stability must be ≥210°C.

(5) Storage stability: No hard settlement after 50°C/7-day accelerated testing (simulating 6 months ambient storage).

(6) Curing compatibility: No significant gel time retardation (≤10% change) or exotherm shift when tested per ISO 2535.

Process optimization involves matching colorant properties to process parameters (mixing speed, time, temperature, resin viscosity) to achieve uniform pigment dispersion and consistent color across production runs.

3. Application Case Study 1: Hand Lay-Up Process Optimization for Marine Composites

A marine composite manufacturer produced boat hulls and decks using hand lay-up with unsaturated polyester resin and glass fiber, experiencing inconsistent color between gel coat and structural layers, and color variation across production batches. The optimization focused on colorant selection, mixing procedure, and gel coat application parameters.

Optimization measures: (1) Selected DENSON unsaturated polyester-based colorant with viscosity 800-1200 mPa·s for gel coat and 500-800 mPa·s for structural resin; (2) Established standardized mixing: pre-mix colorant with resin at 1:1 ratio, 500rpm/5min, then add to full batch at 800rpm/10min; (3) Controlled gel coat thickness at 0.5-0.6mm using a wet film thickness gauge; (4) Implemented color standard calibration with CIE L*a*b* spectrophotometer at each batch start.

Results: batch-to-batch color difference ΔE reduced from 3.5 to ≤0.8, gel coat pinhole rate reduced from 5% to ≤1%, gel coat-to-structural color match ΔE ≤1.0, and production rework rate reduced from 8% to ≤2%. Hull surface gloss (60°) improved from 75GU to ≥85GU.

4. Application Case Study 2: RTM Process Optimization for Automotive Components

An automotive composite manufacturer produced structural components using high-pressure resin transfer molding (HP-RTM) with epoxy resin and carbon fiber, experiencing pigment filtration during resin injection and color streaks on finished parts. The optimization addressed colorant viscosity, particle size, and injection parameters.

Optimization measures: (1) Selected DENSON epoxy-based colorant with ultra-low viscosity ≤300 mPa·s at 25°C and D90 ≤3μm pigment particle size to prevent filtration through fiber preforms; (2) Optimized resin injection temperature at 60°C to maintain mixed viscosity ≤200 mPa·s; (3) Controlled injection pressure at 10-15bar and flow rate at 50-80g/s to ensure uniform resin flow without pigment segregation; (4) Implemented inline static mixing of colorant and resin with 18-element mixer for homogeneous dispersion.

Results: part color difference ΔE ≤0.5, color streak defect rate reduced from 12% to 0%, resin injection time reduced from 120s to 90s, part-to-part color consistency ΔE ≤0.3, and CFRP component flexural strength ≥900MPa (ASTM D790) with no mechanical property degradation from colorant addition.

5. Application Case Study 3: Pultrusion Process Optimization for Structural Profiles

A pultrusion manufacturer produced glass fiber structural profiles using unsaturated polyester resin, experiencing color variation along profile length and pigment settlement in the resin bath during continuous production. The optimization focused on colorant storage stability, resin bath management, and die temperature control.

Optimization measures: (1) Selected DENSON pultrusion-specific colorant with high storage stability (50°C/30-day no hard settlement) and thixotropic properties to prevent pigment settlement; (2) Implemented resin bath agitation at 60rpm continuous circulation with bath temperature controlled at 25±2°C; (3) Controlled die temperature profile: entrance 120°C, middle 150°C, exit 140°C, with line speed at 0.5-1.0m/min; (4) Established resin bath replenishment protocol: add fresh colored resin every 4 hours, complete bath change every 24 hours.

Results: profile lengthwise color variation ΔE reduced from 2.0 to ≤0.5, resin bath usable life extended from 8 hours to 24 hours, profile surface finish Hegman ≥7 grade, Barcol hardness ≥45 (ASTM D2583), and production scrap rate reduced from 6% to ≤1.5%. Color consistency across 1000m continuous production run achieved ΔE ≤0.8.

6. Key Selection Parameters and Usage Recommendations

| Process | Viscosity Requirement | Pigment Size | Key Optimization Parameter | Common Issue |

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

| Hand lay-up | 500-2000 mPa·s | D90 ≤5μm | Mixing procedure, gel coat thickness | Batch color variation |

| RTM/HP-RTM | ≤500 mPa·s | D90 ≤3μm | Injection temp, pressure, flow rate | Pigment filtration, streaks |

| Pultrusion | 1000-5000 mPa·s | D90 ≤5μm | Bath agitation, die temp profile | Lengthwise color variation |

| Filament winding | 300-1000 mPa·s | D90 ≤5μm | Resin bath viscosity, winding speed | Uneven coating |

| SMC | 5000-20000 mPa·s | D90 ≤8μm | Thickening compatibility, storage | Pigment settlement |

Usage recommendations:

(1) Always match colorant viscosity to the specific manufacturing process—using high-viscosity colorant in RTM causes filtration and streaks.

(2) For continuous processes (pultrusion, filament winding), prioritize storage stability and implement resin bath management protocols.

(3) Establish color quality control with CIE L*a*b* spectrophotometer measurements at batch start, mid-run, and end.

(4) Conduct small-scale trials (1-5kg resin) before full production to verify colorant compatibility and curing behavior.

(5) Refer to DENSON composite colorant technical data sheets for process-specific recommendations.

7. Conclusion

The three case studies demonstrate that systematic colorant selection and process optimization significantly improve color consistency and production efficiency across hand lay-up, RTM, and pultrusion processes. The key success factors are: matching colorant viscosity and pigment particle size to process requirements, establishing standardized mixing and application procedures, implementing continuous color quality monitoring, and managing resin bath conditions for continuous production. Composite manufacturers should treat colorant selection as a critical process parameter, not merely an aesthetic choice, and work with colorant suppliers to optimize formulations for their specific manufacturing conditions.

8. FAQ

Q1: What viscosity colorant should be used for different composite manufacturing processes?

A: Colorant viscosity must be matched to the manufacturing process: (1) Hand lay-up and spray-up: 500-2000 mPa·s (Brookfield, 25°C) for easy brush/roller application; (2) RTM and VARTM: ≤500 mPa·s to ensure flow through fiber preforms without filtration; (3) Pultrusion and filament winding: 1000-5000 mPa·s for proper resin bath coating; (4) SMC and BMC: 5000-20000 mPa·s to resist thickening and settlement; (5) Gel coat: 800-1500 mPa·s for proper spray or brush application. Using incorrect viscosity causes defects: too high in RTM causes filtration and streaks; too low in pultrusion causes runoff and uneven coating. Always verify viscosity at the actual processing temperature, as viscosity decreases significantly with temperature (typically 50% reduction per 20°C increase).

Q2: How to prevent pigment settlement in composite resin baths during continuous production?

A: Pigment settlement in resin baths can be prevented through: (1) Selecting colorants with thixotropic additives and surface-treated pigments that provide anti-settling properties; (2) Implementing continuous gentle agitation (30-60rpm) or recirculation of the resin bath to maintain pigment suspension; (3) Controlling resin bath temperature at 20-25°C to prevent viscosity drop that accelerates settlement; (4) Limiting resin bath volume to 4-8 hours of production consumption, with fresh colored resin added periodically; (5) Performing complete bath changes every 24 hours; (6) Using colorants with storage stability certified by 50°C/7-day accelerated testing (no hard settlement). For pultrusion specifically, DENSON pultrusion-specific colorants are formulated with enhanced anti-settling systems for 24-hour bath stability.

Q3: What is the optimal mixing procedure for colorants in composite resin systems?

A: The optimal mixing procedure follows a two-stage approach: Stage 1 (pre-dispersion): Mix colorant with an equal weight of neat resin at 500-800rpm for 3-5 minutes using a propeller mixer, creating a homogeneous color concentrate. Stage 2 (let-down): Add the concentrate to the full resin batch and mix at 800-1200rpm for 5-10 minutes, ensuring uniform color throughout. For high-viscosity systems (SMC, BMC), use a high-shear disperser at ≥2000rpm for 3-5 minutes. Critical rules: (1) Never add colorant directly to full batch without pre-dispersion—causes color streaks; (2) Avoid excessive mixing (>15min) which introduces air bubbles; (3) Add catalyst/accelerator after colorant is fully dispersed; (4) Verify uniformity by drawing a 100μm film on glass and checking for specks or streaks. For gel coat, use high-shear mixing at ≥1500rpm to ensure complete pigment dispersion.

Q4: How does cure temperature affect colorant performance in composites?

A: Cure temperature affects colorant performance through several mechanisms: (1) Pigment thermal degradation: organic pigments may decompose or shift color above their rated temperature (typically 150-250°C), while inorganic pigments are stable to ≥300°C; (2) Carrier resin reactivity: high temperatures can accelerate carrier polymerization, causing viscosity increase and dispersion issues; (3) Cure exotherm: thick sections can reach 30-50°C above mold temperature, potentially exceeding pigment stability limits; (4) Color shift: even without degradation, high-temperature curing can cause temporary or permanent color changes due to pigment crystal structure changes. Selection rule: colorant thermal stability must exceed maximum cure temperature (including exotherm) by ≥30°C. For autoclave curing at 180°C, require colorant stability ≥210°C. Always conduct thermal stability testing by curing colorant-resin samples at actual process conditions and measuring ΔE before/after cure.

Q5: How to establish color quality control in composite manufacturing?

A: A comprehensive color quality control system includes: (1) Incoming colorant inspection: verify viscosity (Brookfield, 25°C), fineness (Hegman gauge, ISO 1524), and tinting strength (ΔE ≤0.5 vs. standard) for each batch; (2) Resin-colorant compatibility test: cure 100g colored resin samples and verify gel time (ISO 2535), Barcol hardness (ASTM D2583), and color stability; (3) Production monitoring: measure CIE L*a*b* color values at batch start, every 2 hours during production, and batch end using a calibrated spectrophotometer (D65 illuminant, 10° observer); (4) Acceptance criteria: batch-to-batch ΔE ≤1.0, within-batch ΔE ≤0.5, part-to-part ΔE ≤0.8; (5) Retain color standard plaques: prepare and store cured color reference plaques under dark, cool conditions for ongoing comparison; (6) Trend analysis: track color values over time to detect gradual shifts from colorant batch variation or process drift. DENSON provides color standard plaques and technical support for establishing QC systems.