Color masterbatches are difficult to manufacture due to difficulties faced in dispersion of colourants. Organic colourants are particularly difficult to disperse in the polymer matrix in non-polar polymers such as PE and PP. Dispersion, however, could be easier in PVC or PS. Wetting agents do improve dispersion but organic colourants are difficult despite incorporation of wetting agents. PE and PP use wax as dispersion agents, which after melting, wets colorants and reduces the agglomeration of colorants which have very fine particle size distribution.
Twin screw compounding, due to higher shear compared to single screw compounding equipment as well as internal mixers, is commonly used in manufacture of masterbatches. Twin screw compounding is used particularly in the developing regions of the world due to their lower capital cost. Higher diameter ratio in twin screw compounding equipment enhances shear and thus dispersion of colourants. Typically diameter ratio of the two screws should be 1.55. This type of equipment also provides economy. Another advantage of twin screw compounding for colour masterbatches is self-cleaning action that such equipment provides. It leaves very little of residue. Due to this action, it is easy to produce a large number of colour masterbatches in different colours in a day, possible uptil 15 different types of colour masterbatches in a day. A quick change of equipment parts could improve such a manufacturing pattern.
While the capital cost of twin screw compounding equipment is much higher compared to single screw compounding equipment and internal mixer, it provides benefits in terms of dispersion of colourants, improving quality of colour masterbatches.
Twin screw compounding equipment also provides tremendous benefits to those masterbatches containing higher loading of fillers such as anti fibrillation masterbatches required for PE and PP raffia / tapes.