SEP Epoxy Color Pastes for Large Electrical Coil Casting and Insulation Systems

A large electrical coil casting is the same epoxy chemistry as electronic potting at a different physical scale, and that scale changes which failures matter. A casting tens of centimetres thick cannot dissipate its cure exotherm, cannot release shrinkage stress through a free surface, and cannot be inspected from outside. In this geometry the colour paste stops being a decorative input: it occupies volume in a thermally and electrically active part, and its pigment chemistry participates in the thermal and dielectric behaviour of the finished casting. The distinction is worth stating plainly. In potting, the dominant risks are solvent entrapment, ionic contamination and cure interference in a sealed but relatively thin volume. In large coil casting, the dominant risks are exotherm control, shrinkage stress and partial discharge in a volume where heat and stress accumulate and where the electric field is designed with a margin that a void can consume.

1. Why scale changes the requirement

Three things change with section thickness. Heat. The cure exotherm generated in the interior has no short path to the surface, so internal temperature rises above the oven set point and the reaction accelerates in the centre. A pigment or carrier that decomposes or volatilises in that window leaves voids exactly where the electric field concentrates. Stress. Shrinkage on cure cannot be accommodated by a thin film, but in a large casting it accumulates into internal stress that later becomes microcracks under thermal cycling. Pigment loading and carrier compatibility affect the shrinkage and the thermal expansion match between resin, conductor and filler. Electric field. In a large coil the insulation is designed with a margin, and partial discharge initiates at the smallest void or the sharpest field concentration. Undispersed pigment aggregates act as field concentration points, which is why fineness and a narrow particle size distribution matter more here than the shade.

2. Pigment chemistry for thick-section castings

Heat resistance is the first screen. The SEP series spans heat resistance from 200 to 300 °C, and in a thick casting the grade should be chosen at the top of that range unless the cure and service temperature have been verified for a lower grade. Pigment content from 15 to 65 percent and imported pigments from Clariant, BASF, DIC Japan and LANXESS Germany are common across the series, so the differentiator at this scale is the heat ceiling and the chemistry of the pigment body. Two inorganic grades sit at the practical end of the series. SEP-3130 is an iron oxide yellow (PY42, 65 percent pigment content, 200 °C heat resistance, LANXESS Germany, 313 type). SEP-3190 is an iron oxide red (PR101, 65 percent pigment content, 300 °C heat resistance, LANXESS Germany, 190 type), the highest heat resistance in the series. The 65 percent pigment content of these two grades is the highest in the series, and that figure matters at this scale for a structural reason: a higher-loading paste introduces less carrier per unit of colour, so less non-resin material has to be accommodated by the curing system and less material can contribute to shrinkage and to volatiles. Iron oxide pigments are chemically inert, low in soluble ionic species and thermally stable well beyond the cure window. For a casting that will run warm for decades, that combination is why inorganic grades are preferred over organic grades whenever the required colour can be reached with them. An organic grade remains a genuine option where the colour cannot be matched inorganically. SEP-9326 is an imported DPP red (PR254, 30 percent pigment content, 260 °C heat resistance, light fastness 8) with high chroma and excellent weather resistance, and it holds a mid-range ceiling. SEP-9328 is a brilliant red (PR170, 25 percent pigment content, 210 °C heat resistance, light fastness 6) with the lowest heat resistance in the series, so its use in a thermally loaded casting has to be justified against the actual cure and service temperature rather than against the colour requirement alone.

3. Colour and thermal behaviour in a thick part

Colour affects how a casting absorbs and emits heat at its surface. A dark casting absorbs radiant heat and reaches a higher surface temperature under the same load; a light casting reflects more and runs cooler at the surface. In thermally rated equipment this is a design variable, not a cosmetic preference, and it should be decided together with the thermal rating rather than after it. The more consequential thermal effect is inside the casting. Pigments that decompose, volatilise or oxidise at service temperature create voids and gas pockets that grow with thermal cycling. Because an opaque casting cannot be re-inspected for shade after cure and cannot be repaired once a void has formed, stability has to be established on aged samples.

4. Electrical considerations

Two pigment-related mechanisms affect dielectric behaviour. The first is ionic. Soluble ionic species in pigment or carrier raise the conductivity of the insulation. Low extractable content is therefore a specification item for a casting grade, not a preference. The second is morphological. A pigment that is not fully dispersed leaves aggregates, and an aggregate is a local discontinuity with a different permittivity from the surrounding resin. Under a high field these discontinuities concentrate stress and can initiate partial discharge. Fineness to GB/T 6753.1 and particle size distribution to ISO 13320 are therefore both release criteria, not production conveniences. A narrow distribution matters as much as a small median, because the largest particle in the distribution sets the worst-case gap. A paste with an excellent median and a coarse upper tail can still fail a partial discharge test.

5. Grade selection for coil casting

Table 1 — SEP grades assessed for thick-section coil casting

Table 2 — Remaining SEP grades for identification and coding

Selection logic: Where thermal margin is the tightest constraint, SEP-3190 at 300 °C with 65 percent pigment content is the reference grade. Where a dark casting with the lowest carrier contribution is required, SEP-989 reaches 300 °C at only 15 percent pigment content. Where surface temperature is a design constraint, SEP-White offers the lightest surface at 260 °C with 50 percent rutile titanium dioxide. Where a yellow oxide shade is required and the thermal envelope is moderate, SEP-3130 matches the inorganic character and loading of SEP-3190 at a 200 °C ceiling. Where an organic red is unavoidable, SEP-9326 at 260 °C is the defensible choice over SEP-9328 at 210 °C.

6. Process practice for large castings

Vacuum and pressure casting. Large coils are normally cast under vacuum and often finished under pressure, so that the compound fills the interstices between conductor, insulation and filler. The paste must not raise viscosity beyond the working window of that process, because an incomplete fill leaves the voids that the vacuum step was meant to remove. Degassing. Vacuum degassing of the paste and of the mixed compound removes entrained air before the fill. In thick sections any residual gas becomes a bubble that cannot migrate to the surface during cure. Stepped cure. Because internal exotherm exceeds the set point, a stepped profile that holds below the peak and completes at final temperature gives a more uniform result than a single hold. The paste must be stable across the full profile, not only at the final temperature. Recalculate the mix ratio. The paste becomes part of the resin phase, so the A to B ratio of a two-pack system has to be recalculated once the paste is added. In a large casting a small stoichiometric shift becomes a large volume of under-cured material. Sectioning and retention. Retain a coupon from each batch and section it after cure to confirm void content and shade uniformity at depth. For a casting that cannot be inspected from outside, this is the only direct evidence.

7. Verification for large castings

Table 3 — Release criteria for thick-section casting grades

8. Frequently asked questions

Q1. What is the main difference between potting and large coil casting for pigment selection?

Section thickness. In potting the dominant risks are solvent entrapment, ionic contamination and cure interference in a sealed thin volume. In a large casting the dominant risks are exotherm control, shrinkage stress and partial discharge in a thick volume.

Q2. Why are iron oxide grades preferred for thick castings?

They are chemically inert, low in soluble ionic species and thermally stable well beyond the cure window, and SEP-3130 and SEP-3190 carry 65 percent pigment content, the highest in the series. A higher-loading paste introduces less carrier per unit of colour, so less material contributes to shrinkage and volatiles.

Q3. Is a 260 °C grade sufficient when the casting cures at 120 °C?

Cure temperature is not the only criterion. The cured casting runs warm in service wherever it carries load, and a shade change or a void cannot be corrected after cure. The ceiling should cover service temperature and thermal cycling, not only the cure step.

Q4. Can an organic red be used in a large casting?

Yes where the colour cannot be matched inorganically, but the grade must be chosen at the top of the organic range. SEP-9326 at 260 °C is the defensible choice over SEP-9328 at 210 °C.

Q5. Why does particle size distribution matter more than fineness alone?

Because the largest particle in the distribution sets the worst-case discontinuity. A paste with a good median and a coarse upper tail can still initiate partial discharge, so both GB/T 6753.1 fineness and ISO 13320 distribution are release criteria.

Key conclusions

1. Large coil casting is the same epoxy chemistry as potting at a different scale, and scale changes the dominant failure modes: exotherm control, shrinkage stress and partial discharge replace solvent entrapment and ionic contamination as the primary concerns.
2. Heat resistance and pigment chemistry are the first screen for thick-section grades. SEP-3190 at 300 °C with 65 percent iron oxide content is the reference grade where thermal margin is tight, and SEP-989 reaches 300 °C at only 15 percent pigment content where a dark, low-carrier casting is required.
3. Dispersion quality becomes an electrical property at this scale. Undispersed aggregates concentrate field stress, so fineness to GB/T 6753.1 and particle size distribution to ISO 13320 are release criteria, and a narrow distribution matters as much as a small median.