Stages A, B and C define the various states of a thermosetting resin (or system).
At stage "A", the base resin and hardener have not been mixed (or have been mixed but have not yet reacted). They have low viscosity, a low mean molecular mass (the resin containing monomers) and total solubility. If stored correctly, they change little or not at all.
At stage"B", the system (resin + hardener) has started to polymerise (prepolymerisation). This is the gelling stage in the prepregs (the pre-impregnated fibres). This stage is not stable and can change (generally very slowly) to stage "C". To avoid this, the reaction is arrested by cooling.
At stage "B" the system:
• has good mouldability (high viscosity but the material is still fusible);
• lends itself easily to shaping and draping (in prepregs);
• is partially soluble (sometimes still totally soluble);
• has a relatively high mean molecular mass with little residual monomer.
At stage "C", the polymer is cross-linked. It is totally insoluble and infusible.