The melt flow index 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. Polymer processors usually correlate the
value of MFI with the polymer grade that they have to choose for different
processes, and most often this value is not accompanied by the units, because it
is taken for granted to be g/10min. Similarly, the test load conditions of
MFI measurement is normally expressed in kilograms rather than any other units.
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 engineer should choose a
material with a melt index high enough that the molten polymer can be easily
formed into the article intended, but low enough that the mechanical strength
of the final article will be sufficient for its use.
Measurement ISO standard 1133-1 governs the
procedure for measurement of the melt flow rate. The procedure for determining
MFI is as follows: A small amount of the polymer sample is
taken in the specially designed MFI apparatus. A die with an opening of
typically around 2 mm diameter is inserted into the apparatus.
The material is packed properly inside the barrel to avoid formation of air
pockets. A piston is introduced which acts as the
medium that causes extrusion of the molten polymer.
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.
After the preheating a specified weight is introduced onto the piston. Examples
of standard weights are 2.16 kg, 5 kg, etc.
The weight exerts a force on the molten polymer and it immediately starts
flowing through the die. A sample of the melt is taken after the
desired period of time and is weighed accurately.
MFI is expressed in grams of polymer per 10 minutes of duration of the test.
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, 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. The flow parameter that is readily
accessible to most processors is the MFI. MFI is often used to determine how
a polymer will process. However MFI takes no account of the shear, shear
rate or shear history and as such is not a good measure of the processing window
of a polymer. It is a single-point viscosity measurement at a relatively
low shear rate and temperature. Earlier, it was often said that MFI give a ‘dot’
when actually what is needed is a ‘plot’ for the polymer processors. However,
this is not true now because of a unique approach developed for estimating the
rheogram merely from the knowledge of the MFI.
The MFI device is not an extruder in the conventional polymer processing sense in
that there is no screw to compress, heat and shear the polymer. MFI additionally
does not take account of long chain branching nor the differences between
shear and elongational rheology. Therefore two polymers with the same MFI
will not behave the same under any given processing conditions.
The relationship between MFI and temperature can be used to obtain the
activation energies for polymers. The activation energies developed from MFI
values has the advantage of simplicity and easy availability. The concept of
obtaining activation energy from MFI can be extended to copolymers as well
wherein there exists an anomalous temperature dependence of melt viscosity
leading to the existence of two distinct values of activation energies for each
copolymer. For a detailed numerical simulation of
the melt flow index, see or. 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;
Part II., Advances in Polymer Technology, Vol. 6, No. 2, pages
125–145. ^ A. V. Shenoy and D. R. Saini:
Thermoplastic Melt Rheology and Processing, Marcel Dekker Inc., New
York. ^ ASTM D1238-04
^ ISO 1133:1997. ^ "ISO 1133-1:2011 Plastics --
Determination of the melt mass-flow rate and melt volume-flow rate of
thermoplastics -- Part 1: Standard method". Retrieved 6 May 2014.
^ A. V. Shenoy, S. Chattopadhyay and V. M. Nadkarni, From melt flow index to
rheogram, Rheol. Acta, Vol. 22, p. 90-101.
^ A. V. Shenoy and D. R. Saini, Upgrading the melt flow index to
rheogram approach in the low shear rate region, J. Appl. Polym. Sci., Vol. 29,
p. 1581-1593. ^ A. V. Shenoy and D. R. Saini,
Re-analysis of extensional flow data of polymer melts, Die Anglewandte Makromol.
Chemie., Vol. 135, p. 77-84. ^ P. Prentice, Rheology and its role in
plastics processing: No. 12, p 25, Section 3.1.3, 1995
^ D. R. Saini and A. V. Shenoy, A new method for the determination of flow
activation energy of polymer melts, J. Macromolecular Sci., Vol. B22, p.
437-449. ^ A. V. Shenoy and D. R. Saini, Effect
of temperature on the flow of copolymer melts, Materials Chemistry Physics, Vol.
19, p. 123-130. ^ Mertz, Alex M., “Understanding Melt
Flow Index and ASTM D1238,” Master's Thesis, University of Wisconsin,
Mechanical Engineering Dept., Madison, WI.
^ Mertz, A.M., A.W. Mix, H.M. Baek and A.J. Giacomin, “Understanding Melt Index
and ASTM D1238,” Journal of Testing and Evaluation, 41(1), 1–13.
ww.prestogroup.commelt-flow-index-deluxe