Masterbatch: how a plastic material is coloured
Polyethylene leaves the petrochemical plant milky white, PET clear. The opaque white of a body lotion bottle, the amber of a syrup, the blue of a cap come from a pellet added to the material at the last moment: the masterbatch.
By Tecmom editorial team · 2 min read
A concentrate in pellets
A masterbatch is a concentrate of pigments or additives, dispersed in a carrier material and then pelletised. The moulder blends it into the natural resin, usually at 1 to 5%, and the machine does the rest: as it melts, the concentrate dilutes through the whole part.
The carrier is often the same polymer as the part, or a compatible one. A poorly chosen carrier can change the properties of the final plastic.
Why a concentrate rather than powder
Handling powdered pigments in a workshop is messy, and dosing them at a few parts per thousand is difficult. A masterbatch solves both problems: it is dosed like a pellet, and its dilution makes dosing more accurate.
It is not the only option. A pre-coloured material, called a compound, can be bought instead, or liquid colours used. The masterbatch keeps the advantage of flexibility: one resin, several colours.
Colour, opacity, protection
A colour is not only about looks. A white pigment makes an HDPE wall opaque, and the tint of an amber bottle filters some of the light that would degrade the medicine. The same method brings non-colouring additives, such as UV stabilisers.
In cosmetics, colour is one of the levers for variants of a single shape: one mould, several colours. Consistency then matters as much as the colour itself: each batch is compared with a standard.
A component of the material, with its own rules
The colourant becomes part of the part's composition, and therefore of what it may release into the content. For food contact, Regulation (EU) No 10/2011 excludes colourants from its list of authorised substances: they fall under national rules, but count towards the overall migration limit. See food contact.
For medicine packaging, the full composition of the material, colourants included, is declared and assessed; see the pharmacopoeia. Changing masterbatch means changing material.
A pack's colour fits in a few pellets. Their composition, however, commits the whole dossier.
Glossary
- Masterbatch
- A pellet concentrated in pigments or additives, diluted into the resin during processing.
- Carrier
- The polymer or wax that holds the masterbatch's pigments.
- Compound
- A material supplied already coloured or with additives, ready to process.
- Overall migration limit
- The maximum quantity of substances a plastic may release into food.