What is the Flow Rate Ratio
Flow Rate Ratio is obtained by dividing an MFI value obtained by use of a higher Mass with one obtained with the use of a lower Mass.
Flow Rate Ratio (FRR) is commonly used as an indication of the way in which the rheological behaviour of a thermoplastic is influenced by the molecular mass distribution of the material. Since polymers are made up of varying lengths of polymer molecular chains, the length of the chain determines the flow characteristics. Since it is difficult to control the exact chain length during polymerisation, the resulting mix or distribution of chain lengths (long and short) also known as the molecular weight distribution, affects the resulting flow properties. This molecular weight distribution is also an important contributor to the physical and mechanical properties of the resulting polymer amongst other items.
According to ISO 1133 (2011), Flow Rate Ratio (FRR) is calculated by taking a mean of Melt Flow Rate measurements using low gravimetric weight (e.g. 2.16kg) and a mean of Melt Flow Rate measurements using a higher gravimetric weight (e.g. 10kg). Dividing one by the other gives a ratio:
FRR = MFR (190 °C, 10kg)/MFR (190 °C, 2.16kg)
Similarly, the ratio can be derived by using the melt volume rate instead of the melt flow rate,
FRR = MVR (190 °C, 10kg)/MVR (190 °C, 2.16kg)
Melt flow index (MFI) or Melt Flow Rate (MFR) is a measure of a given polymers’ flow characteristics also known as the Rheological properties in the molten state under a known applied pressure. The MFI value quoted on many datasheets refers to the amount of polymer that is extruded through a known given orifice (die) and expressed as quantity in g/10 mins or for Melt Volume Rate in cm3/10mins.
The basic principle of MFI is a simple one. Polymer resin, flake or powders are introduced into a heated barrel at the bottom of which is a die with known bore diameter. The standard bore die size is 2.095mm in diameter. It is important to ensure that when the polymer granules are introduced to the barrel that all entrapped air is removed by tampering down the granules, as any air entrapment will give erroneous results. Once the bore is full a piston is placed in the barrel with a known dead weight on top of it. For very basic machines the Extruded samples are cut and the weighed from which the MFI value than calculated.
Why is it Important ?
The viscosity at infinite slow shear is called zero shear rate viscosity (η0) which is essentially what is obtained using the lowest gravimetric weight during MFI. The observed shear thinning of polymer melts is caused by the disentanglement of polymer chains during flow. Polymers with a sufficiently high molecular weight are always entangled and oriented randomly at rest. When sheared, they begin to disentangle and to align which causes the viscosity to drop. The degree of disentanglement will depend on the shear rate. At sufficiently high shear rates the polymers will be completely disentangled and be fully aligned. At this stage, the viscosity of the polymer melt will be independent of the shear rate, i.e. the polymer will behave like a Newtonian liquid again. The same is true for very low shear rates; the polymer chains move so slowly that entanglement does not impede the shear flow.
ISO 1133 – First edition 2011-12-01