Melt flow index

Photograph of an Machine to determine the Melt Flow Rate of thermoplastics according to ISO 1133.

Melt flow index or MFI is a measure of the ease of flow of the melt of a thermoplastic polymer. It is defined as the mass of polymer, in grams, flowing in ten minutes through a capillary of a specific diameter and length by a pressure applied via prescribed alternative gravimetric weights for alternative prescribed temperatures. The method is described in the similar standards ASTM D1238 and ISO 1133.

Melt flow rate is an indirect measure of molecular weight, with high melt flow rate corresponding to low molecular weight. At the same time, melt flow rate is a measure of the ability of the material's melt to flow under pressure. Melt flow rate is inversely proportional to viscosity of the melt at the conditions of the test, though it should be borne in mind that the viscosity for any such material depends on the applied force. Ratios between two melt flow rate values for one material at different gravimetric weights are often used as a measure for the broadness of the molecular weight distribution.

Melt flow rate is very commonly used for polyolefins, polyethylene being measured at 190°C and polypropylene at 230°C. The plastics converter should choose a material with a melt index so high that he can easily form the polymer in the molten state into the article intended, but on the other hand so low that the mechanical strength of the final article will be sufficient for its use.

Measurement

The procedure for determining MFI is as follows:

1. A small amount of the polymer sample (around 4 to 5 grams) is taken in the specially designed MFI apparatus which is nothing but a miniature extruder. The apparatus consists of a small die inserted into the extruder, with the diameter of the die generally being around 2 mm.
2. The material is packed properly inside the extruder barrel to avoid formation of air pockets.
3. A piston is introduced which acts as the medium that causes extrusion of the molten polymer.
4. The sample is preheated for a specified amount of time: 5 min at 190°C for polyethylene and 6 min at 230°C for polypropylene.
5. After the preheating a specified weight is introduced onto the piston. Examples of standard weights are 2.16 kg, 5 kg, etc.
6. The weight exerts a force on the molten polymer and it immediately starts flowing through the die.
7. A sample of the melt is taken after desired period of time and is weighed accurately.
8. MFI is expressed as grams of polymer/10 minutes of flow time.

Synonyms of Melt Flow Index are Melt Flow Rate and Melt Index. More commonly used are their abbreviations: MFI, MFR and MI.

Confusingly, MFR may also indicate "melt flow ratio", the ratio between two melt flow rates at different gravimetric weights. More accurately, this should be reported as FRR (flow rate ratio), or simply flow ratio. FRR is commonly used as an indication of the way in which rheological behavior is influenced by the molecular mass distribution of the material.

References

• A. V. Shenoy, D. R. Saini: Melt Flow Index: More Than Just a Quality Control Parameter. Part I., Advances in Polymer Technology, vol 6, No. 1, pages 1 – 58.

• ISO 1133:1997.

• ASTM D1238-04.

• Various text books on "Polymer Science and Engineering"