What is the meaning of Masterbatch (MB) ?

Masterbatch (MB) is a solid or liquid additive for plastic used for coloring plastics (color masterbatch) or imparting other properties to plastics (additive masterbatch). Masterbatch is a concentrated mixture of pigments and/or additives encapsulated during a heat process into a carrier resin which is then cooled and cut into a granular shape. Masterbatch allows the processor to colour raw polymer economically during the plastics manufacturing process.

The alternatives to using masterbatches are buying a fully compounded material (which may be more expensive and less open to e.g. color variability of the product), or compounding from raw materials on site (which is prone to issues with achieving full dispersion of the colorants and additives, and prone to preparing more material than what is used for the production run). In comparison with pure pigments, masterbatches require more storage space and their lead times are longer. Another disadvantage is additional exposure of heat ("heat history") to both the carrier and the additive; this may be important e.g. for marginally thermally stable pigments.[1]

As masterbatches are already premixed compositions, their use alleviates the issues with the additive or colorant clumping or insufficient dispersion. 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 a way their use is similar to 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 fairly highly concentrated (in comparison with the target composition), with high "let-down ratios"; e.g. one 25 kg bag can be used for a tonne of natural polymer. The relatively dilute nature of masterbatches (in comparison with the raw additives) allows higher accuracy in dosing small amounts of expensive components. The compact nature of the grains of solid masterbatches eliminates problems with dust, otherwise inherent for fine-grained solid additives. Solid masterbatches are also solvent-free, therefore they tend to have longer shelf life as the solvent won't 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 a wax (universal carrier) or on a specific polymer, identical or compatible with the natural polymer used (polymer-specific). E.g. EVA or LDPE can be used as carriers for polyolefins and nylon, polystyrene can be used for ABS, SAN, and sometimes polycarbonates. When a carrier different than the base plastic is used, the carrier material may modify the resulting plastic's properties; where this could be important, the carrier resin has to be specified. The usual ratio of masterbatch to the base resin is 1 to 5 percent. Several masterbatches (color and various additives) can be used together.[1] The carrier can also double as a plasticizer (common for liquid masterbatches) or a processing aid.

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

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

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Choosing and using masterbatch

Masterbatch offers processors an easy and convenient way of adding colour and/or performance enhancing additives to natural or base polymers on their own plant. This technique delivers a compelling combination of cost savings and technical benefits including reduced stock holding of compounded material, reduction in cleaning costs, optimum dispersion and hence reliable colour and performance properties in the finished product.

The benefits can be summarised as follows:

    Low stock holding compared with pre-compounded materials
    Optimum dispersion of pigments/additives
    Dust free, no health and safety risks in comparison to undispersed fine powder pigments or additives
    Elimination of 'trial and error' development or formulation work, as this done by the masterbatch supplier who takes responsibility for colour and consistency
    Reduction in purging/cleaning costs

A masterbatch is a solid concentrate of colour and/or performance enhancing additives such as anti-static, UV stabiliser, metal deactivator, that is added to a base polymer to impart these attributes. It is usually supplied in granular form, either as pellets similar in size to most polymers or as miniature pellets, known as micro granules. Alternatively, masterbatch can be delivered in 'rough cut' form - a mixture of small shreds and chunks.

Two main types are offered - polymer specific and universal. Polymer specific masterbatch is produced using the same base polymer group as the type being used by the processor. Universal masterbatch employs a carrier system that is capable of carrying the high quantities of additives necessary to deliver the end use properties required as well as affording the essential compatibility required across a broad range of polymers.

Universal masterbatches are normally readily available from stock, in small quantities and are cost effective if used correctly. However, in terms of compatibility, they are a compromise and processing difficulties can arise with some materials and engineering polymers that may have to be pre-dried. They are not recommended for use at high dosage levels as the appearance or physical properties of the final product may be adversely affected.

Conversely, polymer specific masterbatch can be pre-dried with the host polymer and added at high loading without compatibility problems. However, only a limited range of colours is available generally and not all engineering polymers lend themselves to conversion to masterbatch.

What to use
End use will always influence the type of masterbatch chosen. Toys and food packaging for example have to be produced from materials made with 'controlled' ingredients and engineering products will demand reliable high performance properties. Typically, an off-the-shelf masterbatch formulation, chosen for colour but incorporating premium priced additives for high specification polymers, when used in an 'uncontrolled' application in a low temperature commodity polymer, would be grossly over-engineered and unnecessarily expensive. Other typical determining factors might be service conditions such as weatherability or light fastness, for example
.
How masterbatch is made
In simple terms, manufacture is a two-stage process: gathering ingredients and then mixing them together by applying heat and shear to produce an homogenised pellet. Generally high or low speed mixers are used to produce a pre-mix of the additives which is then compounded on roller mills, kneaders, single or twin screw extruders. Even distribution (dispersion) and development (breaking down) of the additives throughout the carrier are essential. This processing depends on several key elements: formulation, quality of raw materials, quality of mixing and compounding plant. Finally, the finished masterbatch is pelletised (die-face or strand cut) or ground.

Using masterbatch
Masterbatch can be introduced to the processors' base polymer on the production line in a number of ways, the most successful methods being volumetric or graviametric dosing. In both techniques, the masterbatch is added directly into the polymer stream immediately before processing. Volumetric dosing has the advantage of relatively low capital investment but requires calibrating every time the batch or colour is changed (depending on the bulk density of the material). Graviametric dosing is extremely accurate, but plant is expensive.

Colour; wall thickness and opacity requirements all affect dosage. Although influenced by homogenisation and dosage level, getting the loading right for the production of transparent colours is particularly tricky, as any imperfection is clearly visible. Sometimes it is useful to go for a weaker masterbatch so the demands on homogenisation and dosage are reduced.

End use
Injection moulding, blow moulding, profile extrusion, sheet extrusion, film extrusion, cast film, fibres and calendered sheet processes commonly employ masterbatch.

A basic method of determining whether you will benefit from using masterbatch is to consider that small lots of coloured compound invariably carry a surcharge. Further, if you are using several colours in the same base polymer, especially if it is an engineering polymer, you should explore the cost of using masterbatch. Also, look at the cost of stock holding tonnes of coloured compounded material and weigh this up against using the space saved by installing a plant that contributes to profit. Although masterbatch can cost from £7 to £25 per kilo, it has the potential to save hundreds of pounds per tonne against compounded materials.
Profits can also be improved further through the opportunity to benefit from the bulk purchasing of natural polymers, selecting from possibly a wider range of suppliers. Moreover, there are many different masterbatch suppliers to choose from, so you should be able to find one that offers quality and service to match your exact requirements.

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