Introduction

SEP epoxy color paste is a pre-dispersed pigment concentrate engineered to color and functionalize thermoset epoxy resin systems under sustained high-temperature and high-voltage operating conditions. As power electronics, aerospace composites, and electrical insulation hardware continue to push service envelope boundaries, colorants that merely decorate the resin are no longer sufficient: they must survive peak process excursions near 300 °C, retain dispersion stability through cure and thermal cycling, and preserve the dielectric and mechanical integrity of the host matrix. The SEP series developed by Dongguan DENSON Functional Materials Co., Ltd. addresses this requirement by combining heat-resistant pigment selection, epoxy-compatible carrier resin, and controlled bead-mill dispersion into a single addition that requires no on-site milling.

Figure 1. Representative color shades of pre-dispersed epoxy-compatible pigment pastes.

Conventional colorants introduced as dry pigment powders suffer from incomplete wetting, agglomerate formation, and poor batch-to-batch repeatability when incorporated into filled epoxy systems. Pre-dispersed pastes, by contrast, have already undergone high-shear milling in a resin-carrier medium, reducing the pigment to a controlled particle size distribution and encapsulating each agglomerate with a steric stabilizer (Napper, 1983). This article examines the dispersion mechanism, thermal and electrical performance, and three industrial application cases of the DENSON SEP series, with test data referenced to ASTM, IEC, and ISO methods.

For additional product information on the SEP series and related epoxy color systems, visit the product line page.

Technical Features and Mechanism

SEP refers to a family of solvent-free, epoxy-borne pigment pastes in which the pigment loading typically ranges from 25 wt% to 55 wt%, depending on the tinting strength and density of the chosen pigment. The carrier is a low-viscosity bisphenol-A epoxy resin compatible with most standard liquid epoxy systems, and the dispersion package is formulated with anchoring dispersants that adsorb onto the pigment surface and extend solvated tails into the resin phase. Pigment dispersion stability refers to the resistance of milled pigment particles to flocculation, sedimentation, or color shift during shelf storage and after incorporation into the curing epoxy matrix.

Figure 2. A single-dose SEP epoxy color paste showing a smooth, lump-free viscous consistency after tipping.

The stabilization mechanism follows the steric stabilization model described by Napper (1983) and Parfitt (1981): adsorbed polymer layers create a local osmotic and volume-exclusion barrier when two pigment particles approach, preventing the close-contact van der Waals attraction that drives irreversible flocculation. Because the solvated chains are themselves epoxy-reactive, they become chemically anchored into the cross-linked network during cure, so the dispersion remains locked in place rather than re-agglomerating at elevated temperature. This is distinct from solvent-borne pigment preparations, which rely on electrostatic double-layer stabilization that collapses when the carrier solvent evaporates or is replaced by a thermoset resin.

Three technical features distinguish the SEP series from general-purpose epoxy colorants:

• High-temperature pigment selection. Only pigments with proven heat fastness ≥300 °C short-term (15 min) are selected, including mixed-metal oxide pigments, complex inorganic color pigments (CICPs), and selected phthalocyanine and perylene pigments. Organic pigments with low sublimation temperatures are excluded from high-temperature grades.

• Controlled fineness of grind. The paste is milled to a Hegman fineness value of ≤7 NS (approximately ≤5 µm particle size), measured per ASTM D1316. This fineness ensures color homogeneity in thin-section castings and avoids stress concentrators at the fiber-matrix interface in composite applications.

• Low ionic impurity profile. Raw pigment and dispersant lots are screened for chloride, sodium, and sulfate content to maintain the dielectric properties of the cured epoxy. The target volume resistivity of a 2 wt% loaded casting is ≥1×10¹⁴ Ω·cm, measured per ASTM D257.

During incorporation, the SEP paste is simply stirred into the mixed epoxy-resin-plus-hardener batch at the recommended dosage (typically 1 wt%–5 wt% by weight of the total resin system) using low-shear mixing. No three-roll milling or bead milling is required on the production floor, which reduces equipment investment and avoids introducing additional shear heat into the potting compound.

Application Case 1: High-Voltage Power Module Encapsulation

The first application case is the encapsulation of insulated-gate bipolar transistor (IGBT) power modules used in traction inverters, wind turbine converters, and industrial motor drives. These modules operate continuously at junction temperatures up to 175 °C and experience short-term overload excursions above 200 °C. The encapsulant must simultaneously provide electrical insulation between the high-voltage die and the heat sink, mechanical shock resistance, and moisture protection.

Figure 3. Application of colored epoxy potting compound over an electronic assembly in a controlled manufacturing environment.

The SEP-BK01 black grade is the standard choice for this application. Carbon-black and mixed-oxide black pigments provide complete light blocking, which is important for opto-isolator components integrated into the module, and the high loading improves UV and weathering resistance of the external epoxy shell. At a 3 wt% addition to a standard bisphenol-A / methylhexahydrophthalic anhydride (MHHPA) potting system, the cured compound maintains a dielectric strength of 22–26 kV/mm per ASTM D149 and a glass-transition temperature within 3 °C of the unfilled resin baseline, confirming that the paste does not plasticize or interfere with cross-link density.

Thermal-cycling data from −40 °C to +200 °C (500 cycles, per IEC 60068-2-14) show no cracking, delamination, or color change in the SEP-colored encapsulant. The dispersion remains visually uniform after cure; no pigment bleeding or float-to-surface streaking is observed, which is a common failure mode when low-grade carbon-black pastes migrate during the exotherm peak of fast-curing anhydride systems.

Application Case 2: Aerospace Composite Tooling and Structural Layup

The second case addresses color coding and identification in carbon-fiber reinforced polymer (CFRP) manufacturing. Aerospace shops use colored resin systems to distinguish layup ply orientations, mark inspection zones, or tint semi-transparent resin films for process verification. Autoclave cure cycles typically reach 180 °C for 2 hours, with tooling surface excursions that can approach 250 °C during co-cure of high-temperature prepreg systems such as cyanate ester or bismaleimide (BMI) modified epoxies.

Figure 4. Carbon-fiber composite sheet exiting an autoclave layup line; colored resin systems aid ply orientation identification during layup.

The SEP series offers blue, red, and yellow grades formulated from CICP pigments that remain color-stable after repeated 200 °C post-cure cycles. When added at 1.5 wt% to an epoxy tooling resin, the tinted layup shows uniform color through the part cross-section with no pigment filtration by the carbon-fiber weave — a failure mode known as “pigment stripping” in which large or weakly dispersed pigment particles are mechanically trapped at the fiber bed. The fine Hegman grind (≤7 NS) of the SEP paste minimizes this filtration effect, which is critical for resin-transfer molding (RTM) and vacuum-bag processes where the resin must flow through dense preforms.

Interlaminar shear strength (ILSS) of tinted CFRP specimens, measured per ASTM D2344, is within 5% of the unfilled baseline laminate. This confirms that the dispersed pigment does not weaken the fiber-matrix interface, which would manifest as reduced ILSS and premature delamination under compression-after-impact loading.

Application Case 3: Electrical Insulation and Switchgear Components

The third case is the use of SEP color pastes in cast-resin insulation components for medium-voltage switchgear, busbar supports, and transformer bushings. In these applications, the epoxy casting is the primary electrical insulation between energized conductors and grounded metal hardware, and color coding is used on the factory floor to distinguish voltage classes (e.g., red for high-voltage, yellow for medium-voltage, green for grounding) and to visually inspect casting integrity for voids or inclusions.

Because switchgear enclosures are enclosed and may see elevated ambient temperatures near 60 °C–80 °C in service, and because the casting process itself involves exotherm peaks above 150 °C in thick sections, the colorant must not degrade or release volatile organics that could contribute to tracking or erosion on the epoxy surface. The SEP grades used here are low-VOC and solvent-free, and the pigments pass the glow-wire and Comparative Tracking Index (CTI) requirements of IEC 60112 when incorporated into a filled cast-resin system. The CTI value of the colored casting remains ≥600 V (PLC 1) at the recommended 2 wt% dosage, matching the unfilled resin class.

Figure 5. Programmable environmental chamber used for thermal-cycling and long-term thermal aging qualification of colored epoxy insulation castings.

Long-term thermal aging per IEC 60085 (thermal class B, 130 °C continuous) shows that the SEP-colored castings retain 90% of their flexural strength after 5,000 hours at 155 °C, with no chalking or color fading on the exposed surface. This level of retention is important for insulation components where surface discoloration could mask corona-discharge damage during routine inspection.

Key Selection Parameters and Usage Recommendations

Selecting the correct SEP grade depends on the peak service temperature, required electrical performance, and compatibility with the host resin system. The table below summarizes the typical properties of three representative SEP grades and the test methods used to qualify them. Users should validate against their specific resin formulation and cure schedule, since hardener type (amine, anhydride, cycloaliphatic) can shift the final cure exotherm and Tg.

Usage recommendations: Store SEP pastes in sealed containers at 15–30 °C. Prior to use, homogenize the paste by hand-stirring or slow mechanical mixing; do not apply high-shear dispersers, as this can break the steric-stabilization layer and cause flocculation. Add the paste to the resin component (Part A) before adding the hardener, and stir at low speed (200–400 rpm) for 3–5 minutes to fully incorporate. For high-voltage encapsulation, degas the mixed colored resin under vacuum (≤10 mbar) for 5–10 minutes before pouring to avoid microvoids. For technical data sheets on specific SEP grades, contact the DENSON technical team directly.

Conclusion

The SEP series epoxy color paste delivers pre-dispersed, high-temperature-stable coloring for epoxy systems that must retain dispersion uniformity, dielectric integrity, and thermal resistance under demanding service conditions up to 300 °C peak. By engineering pigment selection, steric dispersion, and a low-ionic epoxy carrier, the SEP line eliminates the need for on-site milling and provides a repeatable color-addition step that preserves the mechanical and electrical performance of the cured resin.

Frequently Asked Questions

Q1: What is the difference between SEP epoxy color paste and dry pigment powder for epoxy coloring?

A1: Dry pigment powder requires on-site milling on a three-roll mill or bead mill to break up agglomerates, which is labor-intensive and introduces batch-to-batch variation. SEP paste arrives pre-milled to a Hegman fineness of ≤7 NS with the pigment already stabilized in an epoxy-compatible carrier. The user simply stirs the paste into the mixed resin; no additional milling equipment is required, and the dispersion remains stable in storage and after cure.

Q2: Can SEP color paste be used in amine-cured and anhydride-cured epoxy systems alike?

A2: Yes. The SEP carrier is a reactive bisphenol-A epoxy resin that participates in cure with both amine and anhydride hardener chemistries. The dispersed pigments and dispersant package are inert and do not catalytically interfere with either cure pathway. In practice, SEP grades have been validated in DETA, DETA, polyamide, methylhexahydrophthalic anhydride, and methyltetrahydrophthalic anhydride systems. A small trial batch is recommended whenever switching hardener chemistry, since cure exotherm profiles differ.

Q3: Does adding SEP color paste reduce the glass-transition temperature (Tg) or mechanical strength of the cured epoxy?

A3: At the recommended dosage of 1 wt%–5 wt%, the measured Tg shift is typically within ±3 °C of the unfilled resin baseline, and flexural strength retention exceeds 95%. Because the paste carrier is itself an epoxy resin that cross-links during cure, it does not introduce a non-reactive plasticizer. At dosages above 8 wt%, pigment crowding can slightly reduce cross-link density and impact strength, so the recommended upper dosage should not be exceeded without validation.

Q4: How should I store SEP color paste and what is its shelf life?

A4: Store the paste in tightly sealed original containers at 15–30 °C, away from direct sunlight and heat sources. Under these conditions, the shelf life is 12 months from the date of manufacture. After opening, reseal the container immediately to prevent skinning or moisture pickup. If the paste shows a thin supernatant oil layer after long storage, remix slowly before use; do not discard it, as the supernatant is reversible resin phase and will reincorporate with gentle stirring.

Q5: Which SEP grade is suitable for continuous service at 200 °C in power electronics encapsulation?

A5: The SEP-BK01 black grade is rated for continuous service at 220 °C and short-term peaks to 300 °C for 15 minutes, making it the default choice for IGBT and power-module encapsulation. The blue and red grades (SEP-BL02, SEP-RD03) are rated for continuous service at 200 °C and are used where color coding rather than light blocking is the primary requirement. All three grades meet the volume-resistivity and dielectric-strength requirements for electrical-grade encapsulation when tested per ASTM D257 and ASTM D149.

References:
Napper, D.H. (1983). Polymeric Stabilization of Colloidal Dispersions. Academic Press, London.
Parfitt, G.D. (1981). Dispersion of Powders in Liquids, 3rd ed. Applied Science Publishers, London.
Ellis, B. (1993). Chemistry and Technology of Epoxy Resins. Blackie Academic & Professional, Glasgow.
Kinloch, A.J. (1988). Adhesion and Adhesives: Science and Technology. Chapman & Hall, New York.
IEC 60085:2007. Thermal evaluation and designation of electrical insulation. International Electrotechnical Commission.
ASTM D149-09(2020). Standard Test Method for Dielectric Breakdown Voltage and Dielectric Strength of Solid Electrical Insulating Materials at Commercial Power Frequencies.
ASTM D257-14(2021). Standard Test Methods for DC Resistance or Conductance of Insulating Materials.
ASTM D1316-20. Standard Test Method for Fineness of Grind of Printing Ink Vehicles.
ASTM D2196-20. Standard Test Methods for Rheological Properties of Non-Newtonian Materials by Rotational Viscometer.
ASTM D2344/D2344M-22. Standard Test Method for Short-Beam Strength of Polymer Matrix Composite Materials and Their Laminates.
ISO 1513:2010. Paints and varnishes — Examination and preparation of test samples.
IEC 60068-2-14:2009. Environmental testing — Test N: Change of temperature.
IEC 60112:2020. Method for the determination of the proof and the comparative tracking indices of solid insulating materials.