Epoxy cast resin

Epoxies are an important class of electronic casting resins. Classic applications include the encapsulation of motors, coils, electronic components, solid-state transformers and long rod insulators in high-voltage cable construction.

When cured, epoxy casting resins are characterized by high glass transition temperatures, good mechanical stability, good electrical properties and excellent chemical resistance. The extreme hardness of epoxies can lead to mechanical and thermal stresses, which are often accompanied by a certain degree of brittleness. The basic brittleness can be reduced by a suitable formulation, but usually cannot be completely avoided.

During processing (usually as a 2-component system), a distinction is made between cold-curing and hot-curing systems. Numerous properties of the epoxies can be specifically influenced by the use of various additives, fillers and other aggregates. The choice of hardener and the curing conditions are particularly important for the desired characteristics.

Epoxies are not only found in casting resins, but also in coatings, adhesives, paints and as construction materials, for example in combination with fibres as fibre composites.

Compared to polyurethanes, epoxy resins are less sensitive to moisture and water. Opened containers can be resealed after use and continue to be used without any loss of quality.

In principle, high curing temperatures of 50°C or more are required for high glass transition temperatures (100-160°C). In order to prevent thermal stress, curing can also be achieved in multi-stage profiles by so-called tempering. Epoxies are primarily composed of bisphenol A/F (Fig. 1).

Cold-curing epoxy resin systems result from the use of aliphatic amines as hardeners (Fig. 2). A lot of heat is released during the curing process, so that high temperatures are reached in the compound. In order to prevent damage to sensitive electronic components, large potting volumes must be moulded in several steps. Hardeners based on aliphatic amines are generally low-viscosity liquids. Cycloaliphatic amines have a lower reactivity towards the epoxy group than aliphatic amines, meaning that catalysers or increased temperatures are required for complete curing.

Processing as a 1-component system is an exception. These are exceptionally inert 2-component systems in which the resin and hardener have already been mixed in advance. The curing reaction only takes place at an elevated temperature. For this reason, the storage of 1-component epoxy systems at low temperatures is essential, as is rapid processing as soon as the compound is heated.

The curing of epoxy resins with carboxylic acid anhydrides or dicyanamide can only be carried out at high temperatures, which is why they belong to the hot-curing systems. Chemically speaking, during the curing process a hydroxyl group reacts with a carboxylic acid anhydride to form a half ester. In the second step, the half ester reacts with an epoxy group to form a diester (Fig. 3).

Increased temperature resistance of epoxy resins is achieved, particularly with heat-curing systems.

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