Monday, February 26, 2018

Melt Flow Index (ASTM D 1238)

To measure flow rate by extrusion Plastometer.
Specimen:
Any form which can be introduced into the cylinder bore may be used. e.g. powder, granules, strips of film. Conditioning required varies with material.
Procedure:
The apparatus is preheated to 190oC for PE. Material is put into the cylinder and the loaded piston (@ 43.25 psi) is put into place. After 5 minute the extrudate issuing from the orifice is cut off, flush and again one minute later. These cuts are discarded. Cuts for the test are taken at 1, 2, 3 or 6 minutes depending on the material or its flow rate. The melt index is calculated and recorded as gm/10 minute.
Significance:
It is primarily useful to raw material manufacturer as a method of controlling material uniformity. The melt index value is strongly indicative of relative “flowability” of various kinds and grades of PE. The ‘property’ measured by this test is basically melt viscosity or ‘rate of shear’. In general, the higher molecular weight materials are more resistant to flow.
Melt index is an inverse measure of molecular weight. Since fl ow characteristics are inversely proportional to the molecular weight, a low-molecular-weight polymer will have a high melt index value and vice versa.
1. Preheat Time. If the cylinder is not preheated for a specified length of time, there is usually some non uniformity in temperature along the walls of the cylinder even though the temperature indicated on the thermometer is close to the set point. The causes the flow rate to vary considerably. There should be zero thermal gradient along the full length of the test chamber.
2. Moisture. Moisture in the material, especially a highly pigmented one, causes bubbles to appear in the extrudate which may not be seen with the naked eye. Frequent weighing of short cuts of the extrudate during the experiments reveals the presence of moisture. The weight of the extrudate is significantly influenced by the presence of the moisture bubbles.
3. Packing. The sample resin in the cylinder must be packed properly by pushing the rod with substantial force to allow the air entrapped between the resin pellets to escape. Once the piston is lowered, the cylinder is sealed off, and no air can escape. This causes variation in the test results.
4. Volume of Sample. To achieve the same response curve repeatedly, the volume of the sample in the cylinder must be kept constant. Any change in sample volume causes the heat input from the cylinder to the material to vary significantly.
Interpretation of Test Results
The melt index values obtained from the test can be interpreted in several different ways. First, a slight variation in the melt index value should not be interpreted as indicating a suspect material. The material supplier should be consulted to determine the expected reproducibility for a particular grade of plastic material. A significantly different melt index value than the control standard may indicate several different things. The material may be of a different grade with a different flow characteristic. It also means that the average molecular weight or the molecular weight distribution of the material is different than the control standard and may have different properties. Melt index is an inverse measure of molecular weight. Since flow characteristics are inversely proportional to the molecular weight, a low-molecular-weight polymer will have a high melt index value and vice versa.

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