Definition
Scorch is partial crosslinking of the compound before shaping is finished – in the Banbury, on the calender, in the extruder or while flowing into the mould. Result: non-flowing stock, unfilled moulds, surface defects and scrap. The Mooney scorch test (t5, 120 °C) and rheometer t2 measure processing safety.
Main causes
Too high a dump temperature or too long a mix; adding the accelerator too early; too fast an accelerator or excess thiuram; heat build-up from high filler/low oil; warm storage; ingredients that activate accelerators, such as resins or PPD derivatives.
How does a retarder work?
CTP (Vulkalent G) scavenges the active mercaptobenzothiazole formed as the sulphenamide breaks down and so delays the onset of cure; the effect is linear with dosage and barely changes optimum cure time. Vulkalent E is the nitrosamine-free sulphonamide type and also works in thiazole/thiuram systems.
Step-by-step fix
1) Lower the dump temperature by 5–10 °C and add the accelerator in a second pass. 2) Try 0.2–0.3 phr Vulkalent G; confirm with rheometer t2. 3) If still insufficient, switch to a more delayed accelerator (MBT→MBTS, CBS→DCBS) or cut thiuram. 4) Limit storage temperature and shelf time.
Why is too much retarder bad?
Above 0.5 phr CTP can bloom, with a slight modulus drop and longer cure. The retarder treats the symptom; the root cause is temperature and the accelerator system.