A B-stage resin is one in which a limited reaction between resin and hardener has been allowed to take place, and the reaction arrested while the product is still fusible and soluble, although having a higher softening point and a more limited solubility than originally. A B-stage resin contains sufficient hardener to effect curing on subsequent heating. The term “B-stage” is one of long standing in connection with phenol formaldehyde resins, where it has a similar, but not a corresponding meaning.
The first epoxy B-stage resin to attract attention was that formed by meta-phenylenediamine, but under controlled conditions it is possible to form B-stage products from a number of bisphenol A resins and hardeners. The most important B-stage resins are produced by using aromatic polyamine hardeners, the B-stage being created by heating the reactants together to bring about a homogeneous state and then cooling.
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