Short answer
This is scorch: the reaction heat in the middle of a large block is high enough to degrade the polymer and some pigments. A colorant cannot cure a formulation scorch problem, but the right pigment choice stops it being made worse or more visible. Pastels and whites show it first. Tell your supplier the block size and water level so heat-stable pigments are specified.
Scorch is driven mainly by foam formulation and block size. A colorant can avoid contributing to it; it cannot prevent it alone.
Likely causes
High exotherm
High water level, large cross-section, high density and warm humid weather all raise core temperature well above 130 °C.
Antioxidant package too light for the exotherm
A foam-formulation matter that sits with your foam chemistry supplier.
Pigments with limited heat stability
Some organic pigments shift in hue at core temperature even when the polymer is fine.
Blocks stacked while still hot
Heat cannot escape, extending time at temperature.
How to diagnose it
- Cut the block mid-section and compare skin to core.
- Log block core temperature on the affected runs.
- Review recent changes in water level, isocyanate index and catalysts.
How to fix it
- Switch to heat-stable pigments for the affected shade.
- Work with your foam supplier on antioxidant package and water and catalyst balance.
- Cool and ventilate blocks before stacking.
How to stop it coming back
- Send your supplier the maximum block cross-section, water level and index with every shade brief.
- Run a full-size block trial for new pastel shades before committing to production.
Where this problem appears most
Send us your batch number, PU system and a photo of the defect and our technical team will help you trace the cause. Ask a technical question.
