Exothermic reactions during epoxy potting: Risks with large volumes
Epoxy resins cure by releasing heat. In thin layers, this is harmless – in large casting volumes, heat can build up and the material can overheat. This leads to discoloration, cracking, and in extreme cases, smoke or charring. This article shows you how to manage this exothermic effect.
To the point
The heat of reaction in epoxy resins tends to build up in thick cross-sections and large containers. Remedies: pour in layers, choose a -exothermic grade , dilute with fillers, cool, and process smaller batches. Silicones and addition-curing systems practically do not have this problem.
Why thick layers are critical
The curing process is exothermic – it releases heat. In a thin layer, this heat is lost to the surroundings. However, in a thick, compact volume, it can be generated faster than it dissipates: The volume-to-surface-area ratio is unfavorable, the core temperature rises, and higher temperatures further accelerate the reaction. This feedback loop can drive the peak temperature far above the ambient temperature – resulting in discoloration, internal stress buildup, cracking, and, in extreme cases, smoke development.
The adjusting screws
| factor | Effect on exothermicity |
|---|---|
| Layer thickness / volume | The thicker it is, the greater the heat build-up |
| Mixing cup size | Large batches heat up in the cup, pot life decreases |
| Resin type | Low-exothermic formulations reduce the peak |
| filler | dilutes the reaction and acts as a heat sink |
| Ambient temperature | Higher = faster reaction = higher peak |
How to control the reaction
Pour large volumes in several layers , allowing each layer to partially set before applying the next. For smaller batches, keep the ambient temperature moderate and use a filled or low-exothermic grade if necessary. Do not mix more than you can use within the pot life – heat builds up most quickly in the container. Basic information on mixing and processing can be found under Epoxy Resin Basics, and related errors under Avoiding Casting Defects and Cracks in the Casting.
Material selection and products
For demanding volumes, we recommend a suitable casting resin type such as the epoxy crystal resin EC 141 or Epocristal and will coordinate the layer structure and batch size with you. Where large volumes with a lower risk of exothermicity are required, an addition-curing silicone or a polyurethane casting resin such as SikaBiresin RE560-93 the more elegant alternative – see Epoxy, Silicone or PU.
Frequently Asked Questions
How thick can I pour epoxy in one pour? This depends heavily on the type; for large volumes, pouring in layers is the safest method. Refer to the volume/layer specifications in the data sheet.
Why is my epoxy getting hot and brown? This is exothermic reaction: the heat of reaction builds up and overheats the material. Smaller batches, thinner layers, and lower ambient temperatures will help.
Does silicone have the same problem? No. Addition-curing silicones react with very little heat generation and are unproblematic for large volumes.
Consultation and samples
Tell us the volume, geometry, and environment – we'll recommend the type and layer structure and provide a sample. Contact us or write to info@silitech.ch.
