SEP Epoxy Color Pastes in APG Casting for Medium and High Voltage Insulation

Automatic pressure gelation is the process behind most medium and high voltage epoxy insulation in service today: 10 to 35 kV solid-embedded poles, post insulators, insulation cylinders, cone insulators and contact boxes. It is a different process from vacuum casting, and the difference is not a matter of equipment. In APG the compound gels inside a heated mould and is demoulded within minutes; in vacuum casting the part is filled cold and cured over hours. That change of thermal history changes what a colour paste has to survive.

1. What APG actually does

Mould temperature is held in the region of 132 to 138 °C. The compound is injected under pressure of roughly 0.32 to 0.38 kg with a flow around 0.9 to 1.1 kg per minute, held under pressure while it gels, and demoulded in a cycle measured in minutes rather than hours. Rapid gelation is the point of the process: it shortens the production cycle to around a quarter of an hour and, more importantly, it controls the exotherm, because the part gels before the exotherm can run away. The advantages claimed for APG components are consistent with that: bubble-free casting, uniform resin density, accurate dimensions, high mechanical strength, low partial discharge and a good surface finish.

2. The constraints this places on a colour paste

Four constraints follow directly from the process. Gel time must not shift. The mould cycle is set by the gel time of the compound. A paste that retards or accelerates gelation moves the demould point, and a part demoulded too early or too late will show voids, internal stress or dimensional drift. In practice this is the constraint that most often rules a paste out, and it is measured rather than assumed. Thermal stability at mould temperature. The paste sits at 132 to 138 °C from injection to demould. A component that volatilises in that window leaves a bubble in a part whose whole selling point is that it is bubble-free. Chemical compatibility with the curing system. Medium and high voltage epoxy casting commonly uses an anhydride hardener such as methyl tetrahydrophthalic anhydride. Anhydride systems are sensitive to acidic species, and many dispersant anchoring groups are acidic by design. The interaction has to be measured as a shift in gel time and a change in cured properties, not reasoned about. Surface quality. A cast insulator has a defined surface finish, and the surface is where the electric field meets the air. A particle standing proud of the surface, or an agglomerate that creates a defect, becomes a field concentration point in exactly the region the design margin was meant to protect.

3. Colour as a functional requirement in switchgear

In switchgear, colour is a safety and inspection function rather than a decorative one. Phase identification depends on consistent colour coding across a production programme. Visual inspection of a cast part looks for voids, flow marks and wall-thickness variation, and all three read more clearly against a defined shade than against an arbitrary one. The shade also has to survive the service environment. Outdoor installations see UV, thermal cycling and the surface contamination that accumulates on insulation over years.

4. SEP grade selection for APG casting

The SEP series is an electronic-grade epoxy colorant system containing no organic solvents, specified for no interference with epoxy curing, migration resistance and stable insulation, with high fineness and narrow particle size distribution. Eleven grades are available with pigment content from 15 to 65 percent, heat resistance from 200 to 300 °C, and imported pigments from Clariant, BASF, DIC Japan and LANXESS Germany.

Table 1 — SEP grades assessed for APG casting

Two selection principles follow. For an outdoor insulator, the inorganic iron oxide grades are the default, because chemical inertness and light fastness matter more than chroma. SEP-3190 at 300 °C and 65 percent loading is the reference grade. For phase identification, the organic grades at 260 °C are the practical choice, but the 210 °C grade requires verification against both the mould temperature and the post-cure schedule.

5. Process practice

Where the paste enters the mix. In an anhydride system the paste should be incorporated into the resin side under controlled mixing, with the hardener and any accelerator added afterwards, so that the gel time can be measured on the actual combination rather than on the resin alone. Gel time measurement. Measure gel time on the compound with and without the paste at the mould temperature. A shift beyond the process window is a stop condition, not a tolerance to be absorbed by adjusting the mould cycle. Degassing. Vacuum degassing before injection removes the air the paste brought in. In APG the part is gel-time limited, so there is no opportunity to let bubbles rise out after filling. Mould temperature uniformity. Because gelation is temperature driven, a mould with uneven temperature produces parts that gel at different times across the cavity. The paste does not cause this, but a paste that shifts gel time narrows the tolerance the mould has to meet. Post-cure. The demoulded part is not fully cured. The post-cure schedule has to be verified with the paste present, because a paste that alters the cure kinetics shifts the final glass transition.

6. Verification checklist

Table 2 — Release criteria for APG casting grades

7. Frequently asked questions

Q1. Why is gel time the first thing to check in APG?

Because the mould cycle is set by it. A paste that shifts gel time changes when the part can be demoulded, and a part demoulded outside that window shows voids, stress or dimensional drift.

Q2. Can an amine-cured paste be used in an anhydride system?

Not without measurement. Anhydride systems are sensitive to acidic species, and the paste has to be evaluated on the actual curing system as a gel time shift and a change in cured properties.

Q3. Is the 200 °C grade acceptable at a 138 °C mould temperature?

The mould temperature alone is covered. The question is the post-cure schedule and the service temperature, and both should be verified before the grade is released.

Q4. Why prefer iron oxide grades for outdoor insulators?

Because chemical inertness, low ionic content and light fastness matter more than chroma in an insulator that will see UV and contamination for decades. SEP-3190 at 300 °C and 65 percent loading is the reference.

Key conclusions

1. APG is defined by its thermal history rather than its equipment. A compound that gels inside a mould at 132 to 138 °C in minutes is subject to constraints that vacuum casting does not impose, and gel time stability is the first of them.
2. Anhydride curing systems are sensitive to acidic species, and dispersant anchoring groups are often acidic. Compatibility has to be measured as a gel time shift and a cured property change on the actual system.
3. For outdoor insulation the inorganic iron oxide grades are the default choice, with SEP-3190 at 300 °C and 65 percent pigment content as the reference and the organic grades reserved for phase identification where thermal verification permits.