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.
B-stage resins can also be produced from aliphatic amines, but because these in general, are more reactive than aromatic amines, the B-stage resins are more difficult to produce. In order to obtain a stable B-stage product it is necessary to use a solid form of the epoxy resin. This may be dissolved in the aliphatic (liquid) amine itself, when after a short time the solid B-stage is assumed. Alternatively the resin and amine may be dissolved in a volatile solvent, but the solvent must be removed rapidly before the irreversible reaction that finally brings about infusibility has advanced very far. Obviously, the highly reactive aliphatic amines such as triethylenetetramine make the preparation of a B-stage material rather difficult, and the product when formed is less stable.
The advantage of B-stage resin in the formation of adhesives is that they enable one component systems to be prepared, especially film adhesives. One such adhesive film is described in which pre-reaction can be affected by either an aromatic diamine or an acid anhydride adhesives.