Viscosity and MFI factors in Polymer Processing

The fundamental principle governing dispersion quality is the viscosity relation between the masterbatch carrier and the host resin matrix.

For optimal dispersion, the masterbatch viscosity should not be too viscous or too fluid

If the masterbatch is too viscous, masterbatch don't melt and blend fast enough. Pigment agglomerates survive the screw without being broken up — you get streaks, specks, and poor color uniformity.

if the masterbatch is too fluid, the masterbatch melts immediately and pools away from the pigment agglomerates before shear forces can act on them. The low-viscosity melt essentially "runs away" from the pigment — dispersion is also poor.

MFI (Melt Flow Index) is the inverse proxy for viscosity, high MFI = low viscosity and vice versa. While viscosity itself is function of temperature as most of non-newtonian fluid in polymer processing, the general approximate relation for formulation most of the time is sufficient.

The general rule of thumb is the MFI of the masterbatch should be higher than the polymer in process or matrix polymer.

What we want here the balance between two type of mixing, the first is dispersive mixing, where breaking agglomerates into primary particles. It requires high shear stress, which comes from a higher viscosity melt. This is why you don't want masterbatch too fluid. and the second is distributive mixing, where the intended additives / colorants spread uniformly across the polymer matrix where a slightly lower viscosity masterbatch relative to the matrix helps here.

The perfect dispersion window means your masterbatch carrier viscosity is tuned to allow both mechanisms to operate — breaking agglomerates first, then distributing them.

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