The Melt Flow Index (MFI) test was an inspired move for many reasons. Primarily, being a low shear rate viscosity test and holding true over many decades of polymer chain lengths, it is a very reliable representative of Average Molecular Weight (AWM). Secondly, it relieved the polymer industry of the formidable problem of making in-process measurements of chain length. AWM is also a reliable measure of tensile strength and other mechanical properties.

The MFItest was first developed in ICI by Messrs. W. G. Oakes and J.C. Greaves. The apparatus consists of a heated barrel, a die, a piston rod and weight. The MFI result is the weight of polymer extruded between two fixed markers and is expressed as g/10minutes.  It was first used for the control of the Low Density Polyethylene reaction more than fifty years ago. This simple manual measurement is now embodied in two international standards, ASTM D1238 and ISO 1133.

The ISO standard Melt Flow Index (MFI) test
Figure 1 The ISO standard Melt Flow Index (MFI) test
Figure 2 Ram Fed Melt Indexer
Computer Fluid Dynamics images of pressure drop and viscosity in the P5 die
Figure 3 Computer Fluid Dynamics image of pressure drop and viscosity in the P5 die

MFI measurement is now used extensively for the main control parameter in many polymer processes. MFI is the principal part of the sales specification for the alpha-olefin polymers.

The basic MFI test is manually operated but can be assisted by a robotic means, provided the test remains true to the standard method. The standard lab test with the 2.16kg weight takes between 6 and approximately 45 minutes to complete plus additional time taken to collect the sample from the plant measurement and to clean down after each test. In the early days the polymer from each unit would be tested every 30 to 60 minutes. It involved a large shift team of laboratory staff to man this 24/7 operation. The test results served the purposes of QC and for reaction control. The MFI Lab test is now very reliable. The ASTM D1238 standard calls for a repeatability of 2%, 95% confidence limits but this is obtainable only with the best class of instrument and when used by fully trained operators. With the advent of computer assisted control the manual Lab test is currently only used for routine spot tests and for urgent process corrections. This combination is not entirely suitable for production in a large scale continuous process.

However, the Lab test is the Standard and it can serve a very useful purpose in verifying calibration (standardisation) of the MFI Analyser.

Replacement of lab test with a process analyser has been tried for nearly as long as the MFI test has been in existence. The basic MFI test has been enhanced and robotised but the evolution towards the process analyser has been brought about by technology that withstands 24/7 operation without frequent maintenance or clean down after each test. The weight, the piston in barrel and the timer have all been replaced by the gear pump and the melt pressure transducer. The molten polymer is supplied either by an extruder or from the solid polymer via a static melter.

Three types of Melt Indexer that employ a capillary die(s) have gained ascendency: –

The On-line main extruder mounted version,

The At-line small extruder fed and the ram fed versions.

Of these three types the ram fed system is an attractive as an MFI analyser because it best deals with the replication of the standard MFItest over a very wide range of polymers. For example, it is possible to obtain spot readings of MFI every 5minutes at a fixed delay of 9minutes with respect to the reaction and with a repeatability of better than 1%, 95% confidence limits for any given sample.  Also the product under test does not suffer from shear damage or thermal degradation to the polymer from extruder action on some of the more sensitive polymers. In particular, finely divided ex-reactor powder materials can be handled without antioxidant making it ideal for direct control of those reactions that are normally considered as inaccessible. Figure 2 shows the schematic of the ram fed Melt Indexer.

Conformance to ASTM D1238/ISO133 Standard

In any control scheme good repeatability of measurement is always helpful but in this project we are meeting more exacting conditions. In this case the polymer is to be manufactured to the specific MFI standard using a control system based on an analyser measurement of MFI. Conformance of the controlling measurement is therefore essential to the project. Conformance of the controlling measurement ensures that the plant remains directly calibrated to the standard. The degree of scatter of the measurement becomes the degree of scatter of the product. Ultimately the plant efficiency and the product quality are tied to the degree of conformance of the measurement.

Close conformance with the ASTM/ISO standards can be achieved in a number of ways. Perhaps the most successful way has been to progressively refine the measurement system of the ram fed Melt Indexer. This has included the whole of the die tract and the operating software. The most recent advancements have used Computer Fluid Dynamic analysis. It has been possible to calculate pressure drops for a number of polymers in the standard MFI apparatus and in various configurations of the measurement head. The design of the flow channel has gone through various iteration cycles involving the entry shape, entry diameter and capillary diameter of the capillary die. The standard L/D ratio and polymer exit to atmospheric pressure, as used in the original design, have been retained. Viable solutions have been found to the critical measurements of pressure and conditioning of the molten polymer to a given temperature, in the capillary section of the die. These advancements have been thoroughly tested in live QC environments at various polymer manufacturing plants and key aspects are covered by MCS IPR. From the early days (1996 onwards) a system of cross-calibration against laboratory MFI results has been developed into a Standard Operating Procedure and is known as a Porpoise Transfer Standard ™. This procedure is used to create a state of maintained calibration in the MFI analyser, where the analyser can be legitimately used to control the plant in conformance with the MFI standard. Recent production trials have shown that the standard error of MFI result can now be a factor of 2 lower than the previous best of 4.5%. The improved conformance is a key payback feature in the project, including the all-important transition periods.