Heat stability is the highest processing temperature a pigment can withstand in a given plastic, for a set time, without an unacceptable change in colour. It matters because plastics are moulded or extruded hot, and many organic pigments start to change colour or break down above a certain point.
The value is not a fixed property of the pigment. It depends on the polymer, the pigment loading, whether white pigment is added, and how long the melt stays hot.
A pigment that fades, darkens or shifts in hue during moulding produces parts that do not match, and the problem may only appear when a machine stops and the melt sits in the barrel. That is why heat stability is one of the first checks for any plastics pigment.
How it is tested
A common method is EN 12877-2, which uses injection moulding. The lab colours the polymer with the pigment and moulds test plaques at a starting temperature. It then raises the temperature in steps, holding the melt in the barrel for a set dwell time at each step, often five minutes.
Each plaque is measured with a spectrophotometer against the plaque from the lowest temperature. The heat stability is the highest temperature at which the colour difference stays within the agreed limit. Ask the supplier which method, polymer, loading, dwell time and colour difference limit it used.
What affects the result
- The polymer matters, since a pigment can be stable in polypropylene but not in polyamide or polycarbonate.
- Full shade and reduced shade behave differently, because added titanium dioxide makes small changes easier to see.
- Low pigment loadings often show lower heat stability, because a small change in dissolved pigment shows more.
- Long residence times, such as large machines or regrind, make the effective heat load higher than the test.
- Some pigments also cause warping or shrinkage in moulded parts, a separate effect seen with some phthalocyanines in polyolefins.
How to specify it
Give the supplier your polymer, your highest processing temperature, your typical residence time and whether you use the pigment in full shade or with white. Ask for heat stability data in that polymer. Build in a safety margin above your process temperature, since barrel zones and hot runners can run hotter than the set point.
Check related properties at the same time. Migration and bleeding matter in soft PVC, light fastness matters for outdoor parts, and food contact rules apply to packaging. Dispersion also matters, as set out in how pigment dispersion is tested. For the Pigment Blue 15 family, read what phthalocyanine pigments are.
Next step
We source pigment grades for your polymer, check supplier test data and have samples tested in your system before you commit. To begin, book a free sourcing review.