1. Introduction

Unsaturated polyester resin (UPR) is the most widely used thermosetting matrix in fiber-reinforced polymer (FRP) composites, accounting for over 40% of global thermoset resin consumption. Coloration of UPR composites presents unique technical challenges: the pigment dispersion must survive free-radical crosslinking, maintain compatibility with low-profile additives (LPA) and thickening agents, and withstand elevated molding temperatures (140-170°C) in SMC/BMC processes. The UP8 series high-performance UPR pigment dispersion, developed by Dongguan DENSON Functional Materials Co., Ltd. (DENSON), addresses these challenges through a styrene-free, filler-free formulation with pigment loading of 40-65%, fineness ≤10 μm, and compatibility verified against Lloyd's Register (LR) marine standards. This article examines the dispersion mechanism, application engineering, and quality control of UP8 dispersions across hand lay-up, SMC/BMC molding, and gel coat processes.

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Figure 1. UP8 series UPR pigment dispersion multi-color product chart

2. Technical Features and Dispersion Mechanism

UPR pigment dispersion is defined as a pre-dispersed concentrate consisting of high-performance pigments stabilized by hyperdispersants in a UPR-compatible carrier, designed for direct addition into unsaturated polyester resin systems. The UP8 series is characterized by five core technical attributes:

2.1 Hyperdispersant Stabilization Mechanism

Pigment dispersion in non-aqueous UPR systems follows the wetting-deaggregation-stabilization model. According to DLVO theory, van der Waals attraction dominates at interparticle distances below 10 nm, requiring steric or electrostatic stabilization to prevent flocculation. Journal of Applied Polymer Science (2025, DOI: 10.1002/app.56144) reported that PEGylated partially hydrolyzed poly(N-vinyl amide) dispersants achieve carbon black particle sizes of 80-150 nm through hydrogen bonding and π-π anchoring, with solvated chains providing 5-20 nm steric barrier. UP8 dispersions utilize polyurethane-type hyperdispersants with aromatic/amine anchoring groups that bind to organic pigment surfaces (phthalocyanines, quinacridones) via π-π stacking and hydrogen bonding, while polyester/polyether solvated chains extend in styrene or acrylate reactive diluents.

2.2 Hansen Solubility Parameter Matching

Long-term dispersion stability requires matching the carrier resin's Hansen solubility parameter (HSP) with the UPR matrix. RSC Advances (2025, 15, 44517-44525, DOI: 10.1039/d5ra07498c) demonstrated that Δδ < 2.0 MPa^0.5 between dispersant shell and matrix is required for thermodynamic compatibility. UP8 carrier resins are formulated with HSP in the 18-20 MPa^0.5 range, closely matching orthophthalic UPR (δ ≈ 19.5 MPa^0.5), ensuring no phase separation during cure.

2.3 Styrene-Free Formulation

Conventional UPR contains 30-40% styrene as reactive diluent, classified as Class 3 flammable liquid under GB 12268-2012. UP8 dispersions use methacrylate-based reactive monomers instead of styrene, eliminating hazardous material classification. ACS Applied Polymer Materials (2026, 8(6), 4573-4586, DOI: 10.1021/acsapm.6c00349) proposed a soft-core/hard-shell gradient copolymer encapsulation strategy for solvent-free pigment systems, where the high-Tg core provides pigment anchoring and the low-Tg shell ensures matrix compatibility—an approach consistent with UP8 carrier design principles.

2.4 Key Performance Specifications

3. Application Case 1: FRP Hand Lay-up and Spray-up

Hand lay-up and spray-up account for approximately 40% of global FRP production. In marine vessel manufacturing, UP8 dispersions are added at 2-5% by resin weight into orthophthalic or isophthalic UPR. A South China yacht manufacturer reported the following improvements after switching from pigment powder mixing to UP8 dispersions:

• Color batch consistency: ΔE reduced from 3-5 to ≤1.0 (spectrophotometer, D65, GB/T 11186.2-1989)

• Gel coat gloss: ≥90 GU (60°, GB/T 9754-2007)

• Color matching time: reduced from 40 min/batch to 8 min/batch

• Gel time deviation: ≤±10% (no cure interference from fillers)

Process parameters: ambient temperature 20-30°C, relative humidity ≤75%, gel time 30-45 min, demolding time 4-6 h. The dispersion is pre-mixed with UPR at 300-500 rpm for 3-5 minutes before adding accelerator (cobalt naphthenate, 0.5-2.0%) and initiator (MEKP, 1.0-2.0%).

Figure 2. FRP hand lay-up manufacturing process

4. Application Case 2: SMC/BMC Molding Compounds

Sheet Molding Compound (SMC) and Bulk Molding Compound (BMC) require pigment dispersions that survive high-temperature compression molding (140-170°C), remain compatible with low-profile additives (polystyrene, PVAc, polyethylene), and do not interfere with MgO thickening. UP8 dispersions at 1-3% loading in SMC and 2-4% in BMC provide:

• Thermal color stability: ΔE ≤1.5 after 160°C molding

• No thickening interference: 24h viscosity reaches 10^7-10^8 mPa·s

• Molded part shrinkage: ≤0.05% (with LPA system)

• Heat deflection temperature (HDT): ≥200°C (GB/T 1634.2-2019)

• Flexural strength: ≥120 MPa (GB/T 1449-2005)

An automotive SMC bumper manufacturer achieved Class A surface finish (Ra ≤0.4 μm) with color matching to body panels at ΔE ≤1.0 after implementing UP8 dispersions and optimized resin paste mixing sequence: UPR → LPA → UP8 dispersion (800 rpm, 5 min) → filler (10 min) → initiator/release agent (3 min) → thickener (2 min).

Figure 3. Colored BMC bulk molding compound

5. Application Case 3: High-Performance Gel Coats

Gel coats serve as the aesthetic and protective surface layer of FRP products, requiring the highest color uniformity and weathering resistance. UP8 weathering-grade dispersions (phthalocyanine blue/green, quinacridone, iron oxide, titanium dioxide) at 3-8% loading in isophthalic/neopentyl glycol gel coat resins provide:

• QUV aging 1000h: ΔE ≤2.0, gloss loss ≤5% (GB/T 1865-2009)

• Chalking resistance: Grade 0 (GB/T 1766-2008)

• Film thickness: 0.4-0.6 mm (spray application)

• Spray viscosity: 15-25 s (Ford Cup #4, 25°C, GB/T 1723-1993)

• Thixotropic index: ≥3.0 (anti-sagging)

Spray parameters: gun orifice 2.0-2.5 mm, air pressure 0.3-0.5 MPa, spray distance 20-30 cm, travel speed 30-50 cm/s with 50% overlap. DENSON technical team provides on-site gel coat color matching support for marine, architectural, and sanitary applications.

Figure 4. FRP tank gel coat spray application

6. Key Selection Parameters and Usage Recommendations

Quality control recommendations: (1) Incoming inspection per batch—color ΔE ≤0.5, fineness ≤10 μm, viscosity within ±20% of nominal; (2) Pre-mix dispersion with UPR before adding accelerator and initiator; (3) Always prepare 50 g laboratory sample with instrument color measurement before scale-up; (4) Compensate for UPR batch base color variation by 0.2-0.5% dispersion adjustment; (5) Store at 5-35°C, use within 12 months, stir thoroughly before use. For technical support, contact Dongguan DENSON Functional Materials Co., Ltd.

7. Conclusion

The UP8 series high-performance UPR pigment dispersion delivers stable coloration across hand lay-up, SMC/BMC molding, and gel coat processes through hyperdispersant steric stabilization, Hansen solubility parameter-matched carrier design, and a styrene-free non-hazardous formulation. With pigment loading of 40-65%, batch color consistency ΔE ≤0.5, and QUV 1000h ΔE ≤2.0, UP8 enables FRP manufacturers to achieve consistent color quality while reducing logistics costs and environmental compliance burden.

8. Frequently Asked Questions (FAQ)

Q1: Why is UP8 styrene-free, and does it affect UPR curing?
A: UP8 uses methacrylate-based reactive monomers instead of styrene, classifying it as non-hazardous for safer transport and storage. These monomers participate in UPR free-radical crosslinking without negatively affecting crosslink density or mechanical properties. Gel time deviation remains within ±10% compared to styrene-based systems.

Q2: What happens if UP8 dosage exceeds 10%?
A: Excessive dispersant above 10% loading may interfere with cure kinetics, prolonging gel time and causing surface tackiness or reduced mechanical properties. Deep colors should be achieved through high-tinting-strength pigments rather than increased dosage. Recommended maximum is 10% for artware casting and 8% for gel coats.

Q3: How does UP8 perform in winter when viscosity increases?
A: UP8 should be stored at 5-35°C. Before winter use, condition at 20-25°C for 24 hours or warm in a water bath (≤40°C) with thorough stirring. Direct high-temperature heating may degrade dispersant structure. For low-temperature curing, increase accelerator by 0.2-0.5% or switch to BPO/tertiary amine cure system.

Q4: Is UP8 compatible with all UPR brands?
A: UP8 shows excellent compatibility with general orthophthalic, isophthalic, and vinyl ester UPR. For specialty modified UPR (high internal mold release, flame-retardant), always conduct a 5% loading compatibility test—observe for flocculation or color floating over 24 hours—before production use.

Q5: What is the shelf life and storage condition for UP8?
A: Shelf life is 12 months unopened at 5-35°C, away from direct sunlight and heat sources. Once opened, use within 3 months and reseal tightly after each use. Products stored beyond 3 months should be stirred at 500 rpm for 5-10 minutes before use. Expired product should be tested for fineness, color, and compatibility before downgraded use.