Certain epoxy adhesives are capable of being "B-staged". A B-staged resin is one in which a limited reaction between the resin and hardener has taken place so that the product is in a semi-cured state. This partial reaction can occur either at room or elevated temperature depending on the formulation. In the B-staged state, the polymeric adhesive is generally a brittle, glass-like solid. However, it is still fusible and soluble in conventional epoxy solvents. On additional heating, the adhesive will progress from the B-stage to a fully cured or crosslinked state. This will usually be accompanied by moderate flow and wetting of the substrate.
The primary advantage of B-staged adhesives is that they permit the formulation of one-component solid adhesives such as films, powders, or preforms. These adhesives are supplied in a form that does not require metering and mixing. Because the end-user is not working with liquid resins or solvents for dilution or clean-up, the B-staged products are considered to be safer and less hazardous than conventional liquid or paste adhesives.