Molecular weight melt flow index test

The melt flow index test method is used to monitor the quality of plastie materials. The quality of the material is indicated in this test by melt flow rate through a specified die under preseribed conditions of temperature, load, and piston position in the barrel as timed measurement is being made. The melt flow rate through a specified capillary die is inversely proportional to the melt viscosity of the material if the melt flow rate is measured under constant load and temperature. The melt viseosity of the material or MFR is related to the moleeular weight of the material if the molecular structures are the same. The extrusion plastometer as specified in... 

Damage can be noticed when melt flow index tests are performed and the reduction in molecular weight causes a higher melt flow value. Also, damage can be noticed when tensile tests are performed and decreases in ultimate elongation are noticed. The intention of this research is to derive a new Qio constant for the Arrhenius Equation to better predict shelf life of medical polymers. 

The standard melt flow index machine is often used for characterising the flow properties of polypropylene and to provide a rough measure of molecular weight. Under the conditions normally employed for polyethylene (2.16 kg load at 190°C) the flow rate is too low for accurate measurement and in practice higher loads, e.g. 10 kg, and/or higher temperatures are used. It has been found that a considerable pressure drop exists in the barrel so that the flow towards the end of a test run is higher than at the beginning. 

The traditional melt index test (230°C/2.16kg for PP) is widely used in the plastics industry to estimate the polymer melt processability by a melt flow index (MFl) which is inversely related to the viscosity and the average molecular weight of the material. The MFI has to be considered very cautiously because polymers are usually processed under very different shear rates and this fairly simple index is far from giving a complete picture of the material s flow behavior. 

The melt-flow index is an extremely simple test, and essentially consists of measuring the amount of polymer melt flowing through a tube of specific dimensions under a specific pressure over a specified amount of time. Experimentally, the apparatus is a small extruder. The index obtained has a sort of inverse relationship to viscosity at any given temperature, and is indirectly related to polymer molecular weight. 

In the polyethylene industry, the molecular weight of polyethylene is measured by determining a Melt Index (MI) value. Melt Index is the amount of molten polyethylene that flows in ten minutes through a specific orifice maintained at 190°C with a weight of 2.16 Kg, positioned directly on the molten polyethylene sample. The test method is ASTM D-1238-condition E and the test value is referred to as the Melt Index, MI or T or simply T. 

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