Masterbatch

A masterbatch (MB) is a mixture of solid additive used for coloring (color masterbatch) or imparting other properties (additive masterbatch) to plastics. Masterbatches are concentrated solid mixtures of pigments and/or additives that are manufactured by encapsulation during a heat process or twin screw extrusion into a carrier matrix (e.g. resin or wax), which is then cooled and processed into granules or left as a bulk solid. Masterbatches can make the coloring of plastics more economical for secondary processors.

Alternatives to using masterbatches include buying an already compounded material (which may be more expensive or less specific to the user's requirements) or compounding the plastic themselves on-site from raw materials.

In comparison to pure pigments, masterbatches require more storage space, and their lead times are longer. Another potential disadvantage is the additional exposure to heat ("heat history") for both the carrier and the additive; this may be important, such as for marginally thermally stable pigments. [1]

As masterbatches are already premixed compositions, their usage can alleviate issues of insufficient dispersion of the additive or colorant when compounding with raw materials. Dispersion can be further modified through the granule size of the masterbatch. The concentration of the additive in the masterbatch is much higher than in the end-use polymer, but the additive is already properly dispersed in the host resin. In this way, their use is similar to the uses of ferroalloys for adding alloying elements to steels.

The use of masterbatches allows the factory to keep stock of fewer grades of the polymer and to buy cheaper natural polymer in bulk.

The masterbatches can be highly concentrated (compared to the target composition), with high "let-down ratios"; for example, 25 kg of masterbatch can be used to compound one ton of natural polymer. The relatively dilute nature of masterbatches, compared to raw additives, allows for higher accuracy in dosing small amounts of expensive additives. The compact nature of the grains in solid masterbatches reduces problems with dust typically inherent to the use of fine-grained solid additives. Solid masterbatches are also solvent-free, consequently tending to have longer shelf lives as the solvent will not evaporate over time. The masterbatch usually contains 40–65% of the additive, but the range can be as wide as 15–80% in extreme cases.[1]

The carrier material of the masterbatch can be based on wax (universal carrier) or a specific polymer that is identical to or compatible with the natural polymer used (polymer-specific). Polymers such as EVA or LDPE can be used as carriers for polyolefins and nylon, and polystyrene can be used for ABS, SAN, and sometimes polycarbonates. When a carrier other than the base plastic is used, the carrier material may modify the resulting plastic's properties, in which case the carrier resin has to be specified. The usual ratio of masterbatch to base resin is 1–5%. Several masterbatches (colors and various additives) can be used together.[1] The carrier can also double as a plasticizer (common for liquid masterbatches) or a processing aid.

Processing machines are usually fed with premixed granules of the host polymer and the masterbatch. The final mixing taking place in the screw and extrusion parts of the processor. This is sometimes prone to adverse effects, such as the separation of the masterbatch and the base material in the hopper. The masterbatch can also be added directly to the machine's screw as a free-flowing solid or, if the masterbatch is liquid, by a peristaltic pump. Such use of liquid masterbatches allows highly accurate dosing and quick color changes between machine runs.[2][3]

Masterbatches can be used in most processes, except rotational molding, Plastisol, and other liquid resin systems.