1. Introduction

Epoxy resins are among the most versatile thermosetting polymers, with global annual production of approximately 3.5-4.0 million metric tons. They are widely used in electronic encapsulation, fiber-reinforced composites, industrial coatings, adhesives, and cast articles. The coloring of epoxy systems is typically achieved through pigment pastes (color concentrates), where the dispersion state of pigments directly affects tinting strength, color consistency, mechanical properties, and electrical insulation performance. Dongguan DENSON Functional Materials Co., Ltd. (DENSON) offers the SEP series of epoxy-based pigment pastes, designed for compatibility across a broad range of epoxy resin types and curing systems. This article examines the dispersion mechanism, curing compatibility, and application parameters of SEP color pastes in bisphenol-A, bisphenol-F, novolac, cycloaliphatic, and glycidyl amine epoxy systems.

For more information on DENSON pigment paste products, please visit the Product Center and the company website.

Figure 1. Laboratory technician stirring blue epoxy color paste, with electronic balance, viscometer, and color chart for quality control.

2. Technical Features and Mechanism

2.1 Dispersion Stabilization Mechanism

Pigment dispersion in epoxy resins follows the three-stage theory: wetting, deagglomeration, and stabilization. The SEP series employs hyperdispersant technology with an anchoring-solvation协同 stabilization mechanism. Anchoring groups (amino, carboxyl, phosphate ester) adsorb onto pigment surfaces through hydrogen bonding, ionic interaction, or coordination, while solvating chains (polyester, polyether, polyacrylate) extend into the epoxy resin carrier, providing steric hindrance to prevent pigment flocculation and re-agglomeration.

Journal of Applied Polymer Science (2025, DOI: 10.1002/app.56144) reported on PEGylated partially hydrolyzed poly(N-vinyl amide) as a carbon black dispersant, demonstrating that an optimal balance between anchoring group density and solvating chain length is critical for dispersion stability. RSC Advances (2025, 15, 44517-44525, DOI: 10.1039/d5ra07498c) reported the synergistic effect of SDBS and polyacrylate terpolymer in stabilizing multilayer graphene aqueous dispersions, confirming that the combination of small-molecule surfactants and polymeric dispersants significantly enhances dispersion stability. ACS Applied Polymer Materials (2026, 8(6), 4573-4586, DOI: 10.1021/acsapm.6c00349) reported soft-core/hard-shell gradient copolymer encapsulation of pigments, where the gradient polymer shell simultaneously provides robust pigment anchoring and matrix compatibility—a principle consistent with the hyperdispersant design in SEP color pastes.

2.2 Curing Compatibility

Epoxy curing proceeds through the ring-opening polymerization of epoxy groups with curing agents. Amine curing agents react via nucleophilic attack of active hydrogens on epoxy groups, while anhydride curing agents undergo alternating copolymerization with epoxy groups under tertiary amine catalysis. The carrier resin in SEP color pastes is a modified epoxy resin with an epoxy equivalent weight (EEW) of 180-220 g/eq, closely matching mainstream bisphenol-A epoxies, ensuring that the paste does not introduce incompatible components or significantly alter curing kinetics.

When added at 5% by weight, SEP color pastes change the gel time of epoxy/amine and epoxy/anhydride systems by no more than 5%, and shift the exothermic peak temperature by no more than 3°C, indicating minimal interference with the curing reaction. The glass transition temperature (Tg) of cured castings differs by no more than 2°C compared to unpigmented controls.

2.3 Compatibility Across Epoxy Types

Different epoxy types exhibit significant variations in molecular structure, polarity, and viscosity, requiring different pigment paste compatibility. Bisphenol-A epoxies (E-51, NPEL-128, DER-331, Epon 828) offer moderate polarity and viscosity (10,000-15,000 mPa·s at 25°C), providing the best universal compatibility. Bisphenol-F epoxies (NPEF-170, DER-354) have lower viscosity (3,000-5,000 mPa·s) but higher aromatic ring density. Novolac epoxies (F-44, DEN-438) have higher functionality (3-6) and viscosity, requiring controlled paste addition to avoid gel time reduction. Cycloaliphatic epoxies (CEL-2021P, ERL-4221) offer excellent weatherability and electrical insulation but lower polarity, necessitating careful dispersant compatibility evaluation.

Table 1. Compatibility of SEP color pastes across epoxy resin types (DENSON laboratory testing, viscosity per GB/T 12007.4-1989).

3. Application Case 1: IGBT Module Encapsulation

IGBT (Insulated Gate Bipolar Transistor) power modules require epoxy encapsulation for electrical insulation, moisture protection, and mechanical support. The encapsulation compound typically uses bisphenol-A epoxy (NPEL-128) with methyl hexahydrophthalic anhydride (MeHHPA) curing agent, BDMA accelerator, and spherical silica filler. SEP-BK black paste (high-color carbon black) is added at 2-4% to achieve complete light blocking.

Formulation and Process Parameters:

Table 2. Reference formulation for IGBT module epoxy encapsulation.

Process要点: Component A (epoxy + filler + paste) is vacuum degassed at 60°C for 30 minutes; Component B (curing agent + accelerator) is preheated at 60°C for 30 minutes. After mixing, the compound is rapidly degassed under vacuum (<1100 pa="">

Figure 2. IGBT power module before and after epoxy encapsulation with SEP-BK black paste.

4. Application Case 2: Carbon Fiber Wound Pressure Vessels

Filament winding is the primary manufacturing process for high-pressure gas cylinders, hydrogen storage tanks, and composite pipes. Carbon fiber is impregnated with epoxy resin and wound onto a liner in a designed pattern, followed by thermal curing. The epoxy matrix typically uses bisphenol-A epoxy (E-51) with methyl tetrahydrophthalic anhydride (MeTHPA) and BDMA accelerator. SEP color pastes are added at 2-5% for product identification and aesthetic purposes.

Key Process Parameters: Resin viscosity is maintained at 300-800 mPa·s at 25°C for proper fiber impregnation. Gel time at 80°C is controlled at 45-90 minutes to provide an adequate winding window. The color paste is pre-mixed with the epoxy resin at 40-50°C for 15 minutes before adding the curing agent. The mixed resin is filtered through a 100-mesh screen to remove pigment agglomerates. Cured composites exhibit interlaminar shear strength ≥50 MPa (GB/T 1450.1-2005) and flexural strength ≥800 MPa (GB/T 1449-2005).

Figure 3. Large pressure vessel undergoing carbon filament winding with colored epoxy matrix.

5. Application Case 3: Epoxy Self-Leveling Flooring

Epoxy self-leveling flooring is widely used in electronic factories, food processing plants, hospitals, and underground parking garages. The topcoat typically employs bisphenol-A epoxy (E-51) with modified cycloaliphatic amine curing agent, reactive diluent (AGE), precipitated barium sulfate filler, and SEP color paste at 4-8% for uniform color.

Performance Requirements: The cured topcoat exhibits gloss ≥85% at 60° (GB/T 9754-2007), adhesion ≥3 MPa (GB/T 5210-2006), abrasion resistance ≤0.03 g at 750g/500r (GB/T 1768-2006), and pencil hardness ≥2H (GB/T 6739-2006). SEP color pastes use weather-resistant pigments (rutile TiO₂, phthalocyanine blue/green, iron oxide red/yellow), with color change ΔE ≤3.0 after 1000 hours of QUV accelerated weathering.

Figure 4. Worker applying yellow epoxy self-leveling flooring in an industrial workshop using SEP color paste.

6. Key Selection Parameters and Usage Recommendations

Table 3. Key technical parameters of SEP series epoxy color pastes.

Usage Recommendations: (1) Stir the paste thoroughly before use; minor settling during storage is normal and does not affect performance after stirring. (2) For cycloaliphatic and glycidyl amine epoxy systems, perform a compatibility trial before batch use. (3) When dosage exceeds 8%, evaluate the impact on mechanical and electrical properties. (4) In brominated flame-retardant epoxy systems, avoid pigments containing copper or iron ions. (5) Store in sealed containers at 5-35°C, away from direct sunlight and moisture.

7. Conclusion

The SEP series epoxy color pastes, through hyperdispersant anchoring-solvation stabilization and epoxy-compatible carrier resin design, demonstrate excellent compatibility and dispersion stability across bisphenol-A, bisphenol-F, novolac, cycloaliphatic, and glycidyl amine epoxy systems. They do not interfere with curing kinetics and have minimal impact on the mechanical and electrical properties of cured products. Selection should be based on specific epoxy grade, curing system, and performance requirements, with compatibility trials recommended for specialty epoxy types.

8. FAQ

Q1: Can SEP color pastes be used in waterborne epoxy systems?

A: The SEP series is designed for solvent-borne and solvent-free epoxy systems. For waterborne epoxy emulsions, DENSON recommends the GT series waterborne color pastes, which use dispersants and carrier resins specifically designed for aqueous environments.

Q2: Why does flocculation sometimes occur in cycloaliphatic epoxy systems?

A: Cycloaliphatic epoxies lack aromatic rings, have lower polarity, and lower viscosity, which weakens the solvation of certain polar anchoring dispersants. We recommend pre-mixing the color paste with an equal amount of cycloaliphatic epoxy, slowly adding it to the main system under high-speed stirring for 5-10 minutes.

Q3: Does high color paste dosage affect electrical insulation performance?

A: At dosages above 10%, some dispersants and carrier resins may slightly reduce volume resistivity. For electronic encapsulation applications, we recommend controlling dosage within 3% and conducting electrical performance verification before use.

Q4: Are SEP color pastes suitable for UV-curable cationic epoxy systems?

A: UV-curable cationic epoxy systems have a different curing mechanism. For UV applications, DENSON recommends the FLUV series UV-curable color pastes, which use dispersants with better stability in cationic initiation systems.

Q5: What is the recommended mixing order for epoxy casting compounds?

A: The recommended order is: epoxy resin → color paste (stir 5-10 min) → filler (stir 15-20 min) → curing agent (stir 3-5 min) → accelerator (stir 2-3 min). The color paste should be thoroughly mixed with the resin before adding fillers and curing agents to avoid local reaction.