Insulating colorant dispersion directly determines the dielectric reliability of cast, potted, and impregnated insulation. This article identifies five process parameters that control fineness and dielectric performance, with measured before-and-after data from epoxy, silicone, and polyurethane production. DENSON insulating colorants from Dongguan DENSON Functional Materials Co., Ltd. are formulated for carrier-matched dispersion.

1. Introduction

Coarse pigment agglomerates and moisture are the two most common causes of reduced dielectric strength and high partial discharge in colored insulation. Controlling the bead-milling process and verifying fineness prevents these defects. The five parameters below—grinding media size, media loading, tip speed, temperature, and exit fineness—were validated across three production lines. See the DENSON EP colorant series for matched grades.

2. Technical Features and Mechanism

Dispersion is the process of wetting pigment agglomerates, breaking them into primary particles by shear and impact in a bead mill, and stabilizing them against re-agglomeration. Residual agglomerates act as field-concentrating defects: the local field around a particle scales with its size and permittivity contrast, so fineness sets the practical breakdown strength.

3. Case Study 1: Epoxy Casting for a Dry-Type Transformer

A transformer maker re-milled a coarse colorant (fineness 12–18 μm) to ≤5 μm. Partial discharge fell from about 20 pC to ≤5 pC per GB/T 1094.11, dielectric strength rose from 18 to 22 kV/mm, and the casting pass rate improved from 82% to 99%.

Fig.1 Horizontal bead mill used for colorant dispersion (process)

4. Case Study 2: Moisture Control in a Silicone Colorant

A silicone colorant absorbed moisture during storage (water content 0.18%). After drying and resealing to ≤0.05%, volume resistivity after 85 °C/85% RH exposure recovered from about 10¹² to ≥10¹⁴ Ω·cm, and the change in loss tangent fell from 0.012 to ≤0.003.

Fig.2 Fineness-of-grind measurement on a Hegman gauge (QC)

5. Case Study 3: Flocculation Control in a Polyurethane Colorant

A polyurethane colorant showed flocculation on an 80-mesh screen and a color difference ΔE of 3.5. Adjusting the dispersant and applying a compatibility check eliminated screen residue, brought ΔE to ≤1.0, and restored dielectric-strength retention to ≥90%.

Fig.3 Dielectric breakdown tester used to verify the cured compound (process)

6. Key Selection Parameters and Recommendations

Recommendation: specify fineness and water content at incoming inspection, require a 48-hour compatibility test, and verify dielectric strength on a cured plaque. More options are on the DENSON website.

7. Conclusion

Controlling media size, loading, tip speed, temperature, and exit fineness brought insulating colorants to ≤5 μm and restored dielectric strength to 22 kV/mm with partial discharge ≤5 pC. Fineness and moisture control are the two highest-leverage process parameters for dielectric reliability.

8. FAQ

Q1: Why does pigment fineness affect dielectric strength?
Coarse agglomerates and the voids around them concentrate the electric field and can initiate partial discharge. Keeping fineness at ≤5 μm per GB/T 1724 (≤1 μm for electronic grades) maintains high dielectric strength and low partial discharge.

Q2: What grinding media size is used for insulating colorants?
Zirconia beads of 0.6–1.0 mm are typical for epoxy and polyurethane colorants, while 0.3–0.6 mm beads suit lower-viscosity silicone systems; media loading is commonly 70–85% with tip speeds of 8–12 m/s.

Q3: How does moisture reduce the performance of a silicone colorant?
Moisture raises ionic conduction and dielectric loss, so after damp-heat exposure volume resistivity can fall toward 10¹² Ω·cm. Drying to a water content ≤0.05% per GB/T 6283 restores resistivity to ≥10¹⁴ Ω·cm.

Q4: How is colorant flocculation detected before production?
A 48-hour compatibility test followed by an 80-mesh screen check per GB/T 6753.1 reveals flocculation. A compatible colorant shows no screen residue, no separation, and a color difference ΔE ≤1.0 per GB/T 11186.

Q5: What incoming-inspection limits should be specified for an insulating colorant?
Specify fineness ≤5 μm, water content ≤0.05%, loss tangent ≤0.020, dielectric-strength retention ≥90%, and a resistivity drop after aging of ≤10%, supported by GB/T 1724, GB/T 6283, GB/T 1409, and GB/T 1408 test reports.