The hardening process has to follow a defined temperature program (examples
given in Fig. 3.8). Several chemical processes happen during curing depending on
the actual temperature [COLL88, GARZ00, p. 331]:
• 70-80 °C: The resin bond starts to flow and to transform into a fused mass. Water in the phenol resin evaporates and the resin hardens under this separation of water [COLL88]. Water can drain off well in porous tools [ESCH05].
• 170-180 °C: The structure finally hardens and the crosslinking of the phenol resin takes place [COLL88]. Overcuring should be avoided because overcured tools exhibit reduced strength [GARZ00, p. 331].
• 180-200 °C: Benzylamin structures split and result in new ammonia generation (Fig. 3.9). The resin bond becomes brittle, but also thermally more stable [COLL88].
End temperature defines bond hardness and toughness
170 — 180 oC Benzylamine structures split
.1..10…-…1..2.0.°.C..Hexamethylene tetramine decomposes
.7.0..-.8.0…0CJBo. nd. s.t. arts…to flow, water evaporates
Steam distillable phenols
• The final temperature level (165-170, 175-180, or 185-195 °C) affects the final tool properties (hardness, toughness, brittleness) considerably [GARZ00, p. 332].
Curing can also happen dielectrically with radio frequency and microwave heating [MENA00, HARI83]. These methods need the existence of a significant electrical loss factor, which is common with phenolformaldehyde resin [HARI83].