Choosing and using masterbatch

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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
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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 gravimetric 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). Gravimetric 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.

Tony Gaukroger
Managing Director
Colour Tone Masterbatch Limited
Pant Glas Farm Industrial Estate
Newport Road
Caerphilly
CF83 8YE UK
Telephone: +44 (0) 2920 888910
Fax: +44 (0) 2920 868487
email@colourtone-masterbatch.co.uk