Melt flow index (MFI) is a measure of the ease of melt flowability of a thermoplastic polymer. The melt flow index is the weight of molten polymer (in grams) that comes out of a die with a certain length and width and under a certain temperature and pressure in 10 minutes according to ASTM D1238 and ISO 11/33 standards.
In fact, it can be defined that the melt flow index is the polymer mass (in grams) that comes out in ten minutes through a hole (Die) with a constant diameter and length at a constant temperature, while A standard weight is placed on the thrust chamber piston which contains about three grams (up to 8 grams) of polymer. The melt flow index is somewhat (but not exactly) inversely proportional to Granory has melt So with the increase Molecular weight Average, decreases. A higher melt flow index indicates more fluidity at processing temperatures. This symbol is originally intended to indicate sylvanic properties as a measure of extrudability. In general, with increasing melt flow index, Tensile strength, tear resistance, softening temperature and Toughness The polymer is reduced.
Today, the following changes have occurred in official documents (standards such as ISO 1133 and ASTM D 1238):
formerly: (MFI = Melt Flow Index) → currently: (MFR = Melt mass-Flow Rate)
formerly: (MVI = Melt Volume Index) → currently: (MVR = Melt Volume-flow Rate)
formerly: (MFR = Melt Flow Ratio) → currently: (FRR = Flow Rate Ratio)
Test conditions depend on the type of polymer. The melt flow index of polymers is an indirect measure of molecular weight that includes the melt flow rate for all thermoplastic polymers of different molecular weights.
Description and method of testing:
First, put the device at the selected temperature for at least 15 minutes.oC190). We fill the cylinder with 3 to 8 grams of sample, depending on the predicted melt flow rate. During the filling of the cylinder, we compact the material by the push rod and by hand. (To ensure the absence of air in samples sensitive to oxidative degradation, finish filling the material within one minute.) According to the melt flow rate of the material, we put the piston inside the cylinder with or without the weight. If the melt mass flow rate is high (for example, more than 10 g/10 min), material loss during the pre-heat time will be significant. In such cases, during the preheating time, we use a piston without weight or a weight less than the desired weight for the test. At the end of the preheating time, we bring the load on the piston to the desired value, i.e. 2.16 kg.
Four minutes after the complete feeding of the material into the cylinder, during which the temperature should reach the selected value, if the piston was not under load before or was under a load less than the desired value, we put the desired load on the piston and let it Let the piston move downwards under the effect of weight so that a thread without bubbles comes out of the device. Depending on the viscosity of the sample, this may be done before or after loading. The time of this operation should not exceed one minute. We cut the thread coming out of the die with a cutting tool and throw it away. Let the piston continue to move down under the force of weight. When the lower mark on the piston reaches the upper edge of the cylinder, start the timer and at the same time cut the output string again with a special cutting tool. Then, according to the mass flow rate, we cut and collect the string at regular intervals. The length of each strand should not be less than 10 mm and more than 20 mm.
When the upper mark on the piston reaches the upper edge of the cylinder, we stop cutting the samples. We ignore the cut samples in which air bubbles are seen. After cooling, we weigh all the remaining cut samples (at least three samples) to the nearest milligram and calculate their average weight. If the difference between the maximum and minimum weight of the samples is more than 15% of their average weight, discard the results and repeat the test with new materials.
The mass flow rate of the melt is calculated with the unit of grams per 10 minutes with the help of the following equation
in which:
C° : Test temperature in degrees Celsius
kg : Nominal mass in kilograms
m : average mass of cut threads in grams
tref : reference time (10 minutes) in seconds (600 seconds)
t : The time intervals of string cutting are in seconds.
Suggested conditions for determining the melt flowability of some common polymers according to the standard ISO و ASTM
The MFI of a polymer material depends on factors such as molecular weight distribution, comonomer percentage, chain branching degree, crystallinity and heat transfer rate in thermoplastic processing. As the MFI of the material increases, the processability becomes easier, but the mechanical properties decrease. In polymer materials, MFI has an inverse relationship with viscosity, in other words, as the amount of MFI increases, the viscosity of the polymer material decreases and vice versa. As the melt flow index (MFI) increases, the tensile strength, tear resistance, softening temperature and toughness of the polymer decrease.
To change the amount of MFI (decrease or increase), MFI increasing or decreasing masterbatch is used. Rakhshan Plimer Sanat Co. is a manufacturer of MFI modifier based on PP and PE.
MFI و MVR
MFI is the flow rate of the polymer melt, expressed in grams per 10 minutes, while MVR (Melt Volumetric Flow Rate) is the volume of melt that flows, expressed as 10.3 min/cm.In fact, MFI can be obtained by multiplying MVR by melt density.
Source:
- McKeen, L. “Introduction to the physical, mechanical, and thermal properties of plastics and elastomers.” The Effect of Sterilization Methods on Plastics and Elastomers; Elsevier: Amsterdam, The Netherlands (2012): 57-84.
- Bremner, T., A. Rudin, and D. G. Cook. “Melt flow index values and molecular weight distributions of commercial thermoplastics.” Journal of Applied Polymer Science41.7‐8 (1990): 1617-1627.
- Shenoy, Aroon. Thermoplastic melt rheology and processing. CRC Press, 1996.
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