Melt Flow Index - Assess the Flowability of Polymers

Among the polymer properties, MFI is used as one of the quality control measurements for thermoplastic polymers, thermoplastic blends, and composites formulation. This is in accordance with ASTM or ISO standards.

Moreover, the MFI value can give you detailed information about the polymers:

• average molecular weight,
• processability, and
• appropriate manufacturing process

A high MFI polymer could be favorable for injection molding. This is because it is easier to fill demanding flow paths in a mold with what is referred to as “high-flow” materials. Low MFI polymers could favor blow molding and extrusion as they deliver higher melt strength. This makes it simpler to control the shape of a parison or a multifaceted profile and die-swell.

Find the solution to all queries related to melt flow index of polymers:

1. What is melt flow index?
2. What is the relationship between MFI and molecular weight?
3. What is the influence of polymer MFI on their processing?
4. How to modify the melt flow index of polymers?
5. What is the effect of melt flow index of the filled polymers/composites?
6. What are the standards for MFI measurements?
7. What are the conditions to determine melt flow index?

What is melt flow index?

The melt flow index (MFI) or melt flow rate (MFR) is a measurement used to assess the flowability or melt viscosity of a polymer. It provides an indication of how easily a polymer can be processed through different processes.

It is defined as the weight of the polymer in grams flowing in 10 min through a die of specific diameter and length by a pressure applied by a given weight at a given temperature. The unit of measurement for MFI is typically grams/10 minutes (g/10 min).

The MFI test equipment is shown in Figure 1. When the volume of the extrudate is measured, the melt volume rate is reported. In polymer processing, the MFI value is correlated with polymer grade. This is done to choose appropriate processing methods.

Figure 1: Melt flow index test apparatus

What is the relationship between MFI and molecular weight?

The MFI of a polymer is inversely proportional to the viscosity. The branching features also affect MFI and MFR values. These features include:

• branching type and
• regularity of the distribution

The broadness of the molecular weight distribution can be estimated via ratios between two melt flow rate values for one material at different gravimetric weights.

MFI is sensitive to several fundamental parameters in polymers, processing, and final product properties. It is essential that the measurement should be performed with extreme care so that possible errors are minimized^[1]. It is important that the potential of MFI is realized by:

• raw material manufacturers,
• polymer processors, and
• product development groups.

It truly is more than just a quality control rheological parameter^[1]. Since molecular weight is one of the properties which determines the performances of the polymers. The melt flow rate is an indirect measure of relative average molecular weight.

The high melt flow rate polymer possesses lower molecular weight. The lower melt flow rate polymer possesses higher molecular weight. Therefore, MFI values are highly beneficial to select the appropriate processing and application of the polymers. Figure 2 shows the empirical relation between linear polypropylene molecular weight and melt flow according to literature data^[2] and equation below:


where,

• MW is the molecular weight expressed in kDalton
• MF is the melt flow measured in standard conditions (230°C and 2.16 kg)

What is the influence of polymer MFI on their processing?

Melt processing of polymer is subjected to shearing and elongational flow. Some processing methods include:

• injection molding,
• compression molding,
• blow molding,
• extrusion,
• thermoforming, etc.

The MFI of a polymer is an important property to select a specific processing method. This is because MFI can be related to shear viscosity, die swell, and elongation viscosity. For example, the calculation of the minimum pressure drops during cavity filling and the minimum clamping force to prevent mold opening during injection molding.

The following parameters can be easily obtained through the knowledge of MFI.

• the compaction force during compression molding,
• the pressure losses through dies of complex cross sections during extrusion, and
• the viscous heat dissipation during processing.

We can monitor MFI measurements using pre- and post-processing operations like:^[1]

• crosslinking
• curing,
• degradation,
• stabilization, and
• aging of the polymer.

Table 1 shows the polypropylene with different MFI values and their processing methods for making different products. For instance,

• Polypropylene with an MFI value of 3.6 g/10 min can be used for monofilament fiber spinning.
• Polypropylene with an MFI of 10 g/10 min can be used to make bulk continuous filament spinning^[2].

Process	MFI (g/10 min)	Products
Fiber spinning	3.6	Monofilament
Injection molding	8.5	Dumb bell
Woven non-woven spun bonding	18	Fabrics
Bulk continuous filament spinning	10.0	Multifilament

Note: The provided MFI ranges are approximate and can vary depending on the grade, manufacturer, and processing conditions. It's always recommended to consult the polymer manufacturer's datasheet or specifications for precise MFI values for a specific grade of polymer.

Important product properties have shown relation to MFI such as:

• tensile strength,
• flexural strength,
• tear strength,
• impact strength,
• tenacity, and
• ultimate elongation.

Lower MFI polymers i.e., higher molecular weight polymers also offer:

• good impact resistance,
• environmental stress-crack resistance,
• fatigue performance,
• barrier properties, etc.

With a proper balance of MFI and processing temperature, a desired level of clarity and gloss can be achieved^[1]. MFI value is a quick guide to select the appropriate processing methods. However, MFI is measured at a constant shear rate and temperature which may not be sufficient to select the processing window of a polymer.

In addition, MFI does not take account of long chain branching, shear rate, shear stress, pressure drop, and temperature gradient^[3]. Therefore, two polymers with the same MFI will not behave the same under any given processing condition^[4].

How to modify the melt flow index of polymers?

It is not easy to find the polymers with required MFI for specific applications. Additives have been used to modify the melt flow index of polyolefin. For example, adding peroxide-based additives can stabilize and increase the MFI of polypropylene.

The advantage of MFI modifying additives are:^[5]

• Extrusion-grade polymer (e.g., polypropylene) can be modified to be used in the injection molding process.
• Limited processing problems connected with the filling of the forming socket.
• Stabilized the MFI indicator and mechanical properties.
• Increased productivity.

What is the effect of melt flow index of the filled polymers/composites?

The incorporation of fillers into the polymers could influence the MFI of the polymers. The addition can vary based on the amount/shape/size of the filler loaded.

• Reinforcing filler improves mechanical, electrical, and thermal properties. Examples: glass fiber, metal powders, etc.
• Non-reinforcing filler helps reduce the cost. Examples: calcium carbonate, talc, etc.

In general, high MFI polymer can help to disperse the filler effectively with good compatibility. While low MFI polymer leads to poor dispersion and insufficient compatibility with fillers.

Effective dispersion with a small amount of filler loading can help to minimize the MFI reduction. Similarly, blending recycled polymers with virgin polymers can also influence the MFI value. Especially when the recycled polymer has a low molecular weight compared to virgin polymer. It is important to measure the MFI value of the filled polymers to select appropriate processing parameters.

Melt flow index of the hygroscopic polymers and fillers

Hygroscopic polymers/fillers are sensitive to moisture absorbed from the atmosphere (Table 2). These materials must be pre-dried to ensure the absorbed water is eliminated. If these materials are not dried enough:

• It could have a detrimental effect on the MFI value.
• The absorbed moisture could lead to an increase in the MFI value by undergoing hydrolysis.

Depending on the polymer/filler, the pre-drying conditions could vary significantly. Therefore, the appropriate drying temperatures and time for each material should be used from the manufacturer’s material datasheets to obtain reliable and reproducible MFI.

Hygroscopic polymers/fibers	Non-hygroscopic polymers/fibers
Polyamides (Nylon)	Polyethylene
Polyesters (PET, PBT, PLA)	Polypropylene
ABS, PC, PMMA	PVC
Polyurethane	Polystyrene
Natural fibers	Glass fibers

Table 2: Hygroscopic and non-hygroscopic polymers and fillers

Melt flow index of recycled polymers and polymer blends:

When polymers are mechanically recycled, they can undergo molecular weight reduction by degradation. Thus, MFI can increase. For example:

• The MFI of the recycled PET bottle (145 g/10min) is five times higher than post-consumer PET flakes (32 g/10 min). This is because of the molecular weight reduction by hydrolytic degradation while mechanically recycling^[6]. Such adverse effect in the MFI of the recycled PET is addressed by the addition of a chain extender. The chain extension also helped to increase the molecular weight of the PET^[7].
• Similar MFI reduction has been observed in biodegradable polyester (PLA). A chain extender is introduced to increase the molecular weight^[8]. Selecting an inappropriate chain extender could lead to an increase in the MFI by acting as a plasticizer^[9].
• Crosslinking additives such as peroxides have also shown to be very effective to reduce the MFI in polyester blends. This is done to tailor the formulations used either in injection molding, blow molding, blown film, or thermoforming products^[10].

What are the standards for MFI measurements?

The following two common standards are widely used for MFI testing in the polymer industry for quality control.

• ASTM D1238 – Standard Test Method for Melt Flow Rates for Thermoplastics
• ISO 1133 – Determination of Melt Mass Flow Rate and Melt Volume Flow Rate of Thermoplastics.

What are the conditions to determine melt flow index?

Table 3 shows the recommended conditions for determination of melt flow rate and melt volume rate for common materials. These are in accordance with International Organization for Standardization (ISO) and American Society for Testing and Materials (ASTM) guidelines. (e.g., 190/2.16=190°C with 2.16 kg weight)

Polymer/Plastic	ASTM Standard Conditions (°C/kg)	ISO Standard Conditions (°C/kg)
Acetals (copolymer and homopolymer)	190/2.16, 190/1.05	190/2.16
Acrylics	230/1.2, 230/3.8	230/3.8
Acrylonitrile-butadiene-styrene	200/5.0, 230/3.8, 220/10	220/10
Acrylonitrile/butadiene/styrene/polycarbonate blends	230/3.8, 250/1.2, 265/3.8, 265/5.0	-
Cellulose esters	190/0.325, 190/2.16, 190/21.60, 210/2.16	-
Ethylene-chlorotrifluoroethylene copolymer	271.5/2.16	-
Ethylene-tetrafluoroethylene copolymer	297/5.0	-
Nylon	275/0.325, 235/1.0, 235/2.16, 235/ 5.0, 275/5.0	-
Perfluoro(ethylene-propylene) copolymer	372/2.16	-
Perfluoroalkoxyalkane	372/5.0	-
Polycaprolactone	125/2.16, 80/2.16	-
Polychlorotrifluorethylene	265/12.5	-
Polyethersulfone	380/2.16, 360/10, 343/2.16	-
Polyethylene	125/0.325, 125/2.16, 250/1.2, 190/0.325, 190/2.16, 190/21.60, 190/10, 310/12.5	190/2.16, 190/21.6, 190/0.325, 190/5
Polycarbonate	300/1.2	300/1.2
Polypropylene	230/2.16	230/2.16
Polyphenylsulfone	365/5.0, 380/2.16	-
Polystyrene	200/5.0, 230/1.2, 230/3.8, 190/5.0	200/5
Polysulfone	343/2.16, 360/10	-
Polyterephthalate (PET, PBT)	250/2.16, 210/2.16, 285/2.16	-
Poly(vinyl acetal)	150/21.6	-
Poly(vinylidene fluoride)	230/21.6, 230/5.0	-
Poly(phenylene sulfide)	315/5.0	-
Styrene acrylonitrile	220/10, 230/10, 230/3.8	220/10
Styrenic thermoplastic elastomer	190/2.16, 200/5.0	-
Thermoplastic elastomer-ether-ester	190/2.16, 220/2.16, 230/2.16, 240/2.16, 250/2.16	-
Thermoplastic elastomers	230/2.16	-

References

1. Shenoy, A. V., and D. R. Saini. "Melt flow index: More than just a quality control rheological parameter. Part I." Advances in Polymer Technology 6.1 (1986): 1-58.
2. Fambri, Luca, and Luca Lutterotti. "Effect of Processing and orientation on structural and mechanical properties of polypropylene products." Polypropylene-Polymerization and Characterization of Mechanical and Thermal Properties. IntechOpen, 2019.
3. Shenoy, A. V., and D. R. Saini. "Re‐analysis of extensional flow data of polymer melts." Die Angewandte Makromolekulare Chemie: Applied Macromolecular Chemistry and Physics 135.1 (1985): 77-84.
4. P. Prentice, Rheology and its role in plastics processing: No. 12, p 25, Section 3.1.3, 1995
5. https://www.bedeko-europe.com/peroxide-modifier-mfi-modifier-of-polypropylene/ (accessed on 05/06/2023)
6. Gupta, Arvind, Manjusri Misra, and Amar K. Mohanty. "Novel sustainable materials from waste plastics: compatibilized blend from discarded bale wrap and plastic bottles." RSC advances 11.15 (2021): 8594-8605.
7. Wu, Wen-Jun, et al. "Recycled Poly (Ethylene Terephthalate) from Waste Textiles with Improved Thermal and Rheological Properties by Chain Extension." Polymers 14.3 (2022): 510.
8. Barletta, Massimiliano, Clizia Aversa, and Michela Puopolo. "Recycling of PLA‐based bioplastics: The role of chain‐extenders in twin‐screw extrusion compounding and cast extrusion of sheets." Journal of Applied Polymer Science 137.42 (2020): 49292.
9. Correia, Carlos, et al. "Reprocessability of PLA through chain extension for fused filament fabrication." Journal of Manufacturing and Materials Processing 6.1 (2022): 26.
10. Mohanty, Amar Kumar, Manjusri Misra, and Feng Wu. "Biodegradable nanostructured composites." U.S. Patent No. 11,279,823. 22 Mar. 2022.