‘B-stage’ indicates an intermediate stage in the curing of a thermosetting resin. Some cross-linking has occurred, but the process is not yet complete, and the resin can be heated and caused to flow, allowing it to be finally cured in the intended shape. This property of certain thermosets is very important for electronics applications, the reactivity of B-stage resins making possible the manufacture of multilayer PCBs.
The description of the material as a epoxy indicates both the presence of epoxide groups in the structure, and the fact that the resin is a thermoset, but the material might be any one of a wide range of resins, whose mechanical properties will depend as much on the fillers as on the resin binder.
Compared with a finished moulding, the B-stage resin will be able to be moulded, especially when heat is applied, and it will be much less hard and rigid and not resistant to solvent attack. B-stage is also not a stable condition, and the curing mechanism will continue, at a rate which depends on the conditions. For this reason some epoxy resins and laminating fabrics have to be kept refrigerated until ready to use, in order to have an acceptable working life.
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