MELT INDEX: AN IMPORTANT (BUT NOT EXHAUSTIVE) INDICATION OF THE MELT VISCOSITY FOR A THERMOPLASTIC MATERIAL
Melt index or fluidity index is one of the most commonly used properties to provide an indication of the fluidity of a thermoplastic material.
Test method is described in standards such as ASTM D1238 and ISO 1133 and, conceptually, it is a relatively simple test: a sample of about 5 grams of thermoplastic material (or compound based on thermoplastic material) is heated above the melting (or softening) point and forced to flow through a capillary of known size using a piston actuated by a defined weight (generally 2.16 or 5kg).
The weight of the material flowed through the capillary in 10 minutes is then evaluated.
Fig.1: Schematic representation of the test apparatus (Melt Flow index tester).
This value, expressed in [grams / 10 minutes], is the value of Melt Flow Index (it’s often used the acronym MFI) also known as Melt Flow Rate (whose acronym is MFR) or grade of the material.
We also often speak of MVR (acronym of Melt Volume Rate) whose value is expressed in [cm3 / 10 minutes].
The relationship between the MFI and MVR is the density of the material at test conditions (density of the melt), that is: MFI [g / 10 '] = MVR [cm3 / 10'] x melt density [g / cm3].
A high MFI value therefore indicates a low viscosity of the melt at test conditions and vice versa (if much material flows the MFI value is high and the viscosity of the melt, at test conditions, is low and vice versa).
The MFI value is therefore inversely proportional to the viscosity of the melt at test conditions.
This property (MFI, MVR) is often used to evaluate the constancy (quality) of the material of different batches and is used also for the preselection of a material (especially for polyolefins MFI is a property often reported on the data sheet).
The MFI value is also specificallyuseful to determine whether a material has undergone a degradation process (which changes the molecular weight and therefore the viscosity of the polymer) during transformation (for example during the injection moulding of a finished part).
However, it should be taken into account that the MFI value, although it is an important indication of the melt viscosity of a thermoplastic material, represents only a single viscosity point (value) of what is the entire rheological curve of the material (not considering the fact that passing from the MFI value to the viscosity value is feasible only through approximate correlations).
The MFI value therefore indicates the specific viscosity value of the melt obtained under specific conditions (test conditions), that is the value obtained at a given temperature and at a given deformation rate (determined by the weight applied, for the geometry defined by test method).
Generally, when speaking about the rheological curve (Fig. 2) we consider the curve that represents the viscosity of the melt, generally indicated with the symbol [η] ("eta") and expressed in [Pa * s], depending on the deformation rate generally indicated with the term shear rate and with the symbol [γ *] ("gamma point") and expressed in [1 / s].
At different temperatures are clearly obtained different rheological curves. (Fig.2) considering that the viscosity of the melt depends significantly (exponentially) by the temperature.
Depending on the specific production process (extrusion, injection moulding, blow moulding, etc.) the shear rate values involved can be significantly different and so the viscosity values of the material will be as well, considering the non-Newtonian behaviour of the polymeric materials (considering specifically, pseudoplastic or "shear thinning" materials).
In general terms, for example, the shear rate range for extrusion processes is in the order of 102 -103 [1 / s] while for injection moulding processes we speak of shear rate range in the order of 103 -104 [1 /s]
The different test methods, in turn, allow an evaluation of the viscosity in different shear rate ranges: with the MFI, shear rate values are in the order of tens of [1 / s] (depending on the weight applied), with capillary rheometry shear rate values are in the order of tens of thousands of [1 / s ] while with rotational rheometry it is possible to reach very small values of shear rate (in the order of 10-2 [1 /s] ).
Fig.2: Example of rheological curve of a pseudoplastic material (viscosity vs shear rate in bilogarithmic scale).
This implies that the MFI value may not be indicative of the behaviour of the material in a specific process as well as it may make two materials appear to be similar in terms of fluidity even if in fact they are not or assume that a material is more fluid (less viscous) of another when the opposite is true in the shear rate interval of specific interest.
Fig.3: Rheological curve of generic materials.
For example, considering the curves shown in Fig. 3, it can be seen that materials A and B, despite having the same MFI value, have significantly different melt viscosity at high shear rate values (material A has a lower viscosity than material B at high shear rate).
On the other hand, material C, despite having a lower viscosity (higher MFI) than material A (and B) for the shear rate value corresponding to the MFI, has a higher viscosity than material A (and B) at high shear rates.
It must be said that the determination and evaluation of the rheological curve is not always simple or immediate.
In RDLab137 we can support in the determination of the rheological curve, at different temperatures, with various techniques such as rotational rheometry and capillary rheometry, and therefore to evaluate (and compare) in a more complete way the rheological behaviour of materials of interest.
Ing. Luca Ciceri - RDLAB137 srl
Last revision: 20/12/2020
