Rather than handling raw additive powders or pigments directly, which can be difficult to meter accurately and potentially hazardous to handle in pure form, the film industry pre-disperses these materials into a carrier resin at a high concentration, producing pellets called masterbatch. A color masterbatch, for example, might contain 25 to 50 percent pigment loaded into a polyethylene carrier, and during extrusion a small percentage of that masterbatch, often just 2 to 5 percent of the total resin feed, is blended with virgin base resin to achieve the desired final color throughout the film.
The same concept applies to functional additives like slip agents, anti-block additives, UV stabilizers, and antistatic agents, all of which are typically supplied as masterbatches rather than raw chemicals to be added at the extruder. This approach gives much more consistent and accurate dosing than trying to meter tiny percentages of raw additive directly, and it keeps the base resin supply chain simple since the same virgin resin can be used across many different finished products just by swapping which masterbatches get blended in at the extruder.
Masterbatch selection and dosing rate are core parts of formulating any film recipe, and getting the let-down ratio (the ratio of masterbatch to base resin) right is essential, since under-dosing won't deliver the intended color or function, while over-dosing wastes material and can negatively affect other film properties like clarity or sealing.
Glossary
Masterbatch
A concentrated pellet form of additives or color pigment carried in a resin base, blended into base resin at a small percentage during extrusion.
Example
A converter producing a black trash liner will blend a black color masterbatch into the LLDPE or LDPE base resin at the extruder, typically at a let-down ratio specified by the masterbatch supplier, rather than using pre-colored resin pellets.
Nuance
People sometimes assume masterbatch refers only to color, but the term covers any concentrated additive package, including slip, anti-block, UV stabilization, and antistatic formulations. When a spec sheet references a masterbatch without further detail, it's worth clarifying whether it refers to color, a functional additive, or a combination package.