Melt flow-index

1. Melt Flow Index
EMA 4161C
PHYSICAL PROPERTIES OF POLYMERS
MELT FLOW INDEX LABORATORY
PURPOSE:
To familiarize the student with the operation of an extrusion plastometer, and
demonstrate the relationship between the melt flow index (MFI) and molar mass.
OVERVIEW:
The extrusion plastometer is a simple form of capillary rheometer, which is commonly
used to compare different grades of the same polymer type. The plastometer itself is
simply a heated cylinder that is used to extrude a polymer melt. The cylinder is charged
with polymer and held for sufficient time to ensure complete melting. A piston carrying
a static weight then loads the charge from above. As the load on the piston drives the
melt through a die at the bottom of the cylinder, sections of the extrudate are cut-off at
measured time intervals. By definition, the MFI is the mass of extrudate driven through
the die in a 10-minute interval. Therefore, the index is an indirect measure of viscosity at
specified load/temperature combinations.
The principal difference between grades of the same polymer that is reflected in the MFI
is molar mass. Because of the inverse proportionality between the MFI and viscosity, in
general, polymer grades with a higher melt flow index possess lower molar mass.
However, the presence of additives, e.g., plasticizers and impurities, can also affect
viscosity and consequently, the MFI. The melt flow index is commonly used by
suppliers of polymer stock and those in the injection molding or extrusion industries to
determine processing conditions for different grades of polymer. For instance, a
manufacturer of computer keyboards, which are injection molded, might increase the
injection pressure, mold temperature, or hold time for a grade of ABS with a lower than
average melt flow index.
MATERIALS:
 ~5g of Dow Styron polystyrene
 ~5g of Dow acrylonitrile-butadiene-styrene (ABS)
 ~5g of Philips 66 HLN-120-01
 ~5g of Philips 66 HLN-200
 ~5g of Philips 66 HLN-350
EQUIPMENT:
 Tinius-Olsen extrusion plastometer and accessories
 “Scoopula” or a pair of scissors to manually cut the extrudate
 Small beakers
 Electronic balance
 High temperature gloves
 Safety glasses
PROCEDURE:

2. Melt Flow Index
1. Turn on the extrusion plastometer and place the plunger in the sample cylinder. Set
the initial operating temperature at 200 °C.
2. Allow 15-20 min to stabilize temperature at precisely 200 °C.
3. Weigh ca. 5g of both PS and ABS into small beakers.
4. Once the plastometer is thermally stable, pour 5g of PS into the plastometer sample
cylinder, and replace the plunger into the cylinder atop the PS sample. (CAUTION:
The top of the sample cylinder is ca. 200 °C so do not touch the cylinder)
5. Place the 4900g weight atop the plunger, and allow 6-8 min for the PS to melt and
produce a bubble-free extrudate (longer preheat time may be required).
6. Cut-off the generated extrudate, and allow the bubble-free extrudate to be produced
for the 3 min.
7. Once again, cut-off the extrudate and weigh this sample.
8. Continue taking samples in 3 min. intervals, until no more extrudate is produced,
weigh and record each (make sure to keep track of the order in which these samples
are taken).
9. Calculate the amount of extrudate that would have been produced in 10 min. This
amount per 10 min is the melt flow index for the tested temperature/load
combination.
10. Clean-out the sample cylinder using the provided bore cleaner.
11. Repeat the same process for the ABS.
12. Ramp the temperature of the plastometer up to 230 °C, and allow 20-15 min for the
instrument to stabilize.
13. Repeat the same procedure for the 3 PP samples, with the exception that a 2060g load
will be used for testing.
14. Ramp the instrument down to 25 °C, and turn the instrument off.
15. Clean up the workspace, plunger, and die.
ANALYSIS:
Determine the average melt flow index and sample standard deviation for each of
the 5 different polymers tested above. Tabulate the polymers tested, load/temperature
combinations and MFI values.
QUESTIONS:
1. Does there appear to be a trend indicating a dependency of the melt flow index on the
order in which the samples for a given polymer were obtained?
2. Compare the MFI of the PS with that of the ABS. What structural or morphological
aspects of each polymer give rise to the differences?
3. Compare the MFI of the 3 different grades of PP. What are the factors that give rise
to the radically different MFI values for these 3 samples of the same polymer? (Hint:
discuss the effect of additives, and the molar mass requirements for each of the
intended methods of processing the three polymers)
4. Can MFI values of two different polymers, each obtained under a different
temperature/load combination, be used to compare the apparent viscosities of the two
polymers?
REFERENCES:

3. Melt Flow Index
1. Morton-Jones, D.H., Polymer Processing, Chapman and Hall, New York, (1989), pp.
44-45.
2. ASTM D 1238-90b.
EMA 4666C - POLYMER PROCESSING LABORATORY
EXPERIMENT 2 - MELT FLOW INDEX
SUPPLEMENTAL TABLE
This sheet provides a listing of the polymers and temperature/load combinations found to be satisfactory by
ASTM.
Polymer
Temp (°C) /
Load (kg)
Sample
Mass (g)
Charge Time
w/o Load (sec)
Charge Time
w / Load
(sec)
Sampling
Time (sec)
Polystyrene 200 / 5 3.5 180 180 180
ABS 200 / 5 4 180 180 120
PP
(Ext. Grade)
230 / 2.16 3.2 180 180 120
PP
(Gen. Purp.)
230 / 2.16 5 240 120 15
PP
(Inj. Mold. Gr. )
230 / 2.16 5 300 60 15