Viscosities and Melt Flows

Viscosity is a measure of resistance to plastic melt flow. It is the internal friction in a melt resulting when one layer of fluid is caused to move in

Figure 1,6 Examples of narrow and wide molecular weight distributions

Figure 1,6 Examples of narrow and wide molecular weight distributions

LOW INCREASING MOLECULAR WEIGHT HIGH

WIDTH

LOW INCREASING MOLECULAR WEIGHT HIGH

WIDTH

relationship to another layer. Thus viscosity is the property of the resistance of flow exhibited within a body of material. It is the constant ratio of shearing stress to the rate of shear. Shearing is the motion of a fluid, layer by layer, like playing cards in a deck. When plastics flow through straight tubes or channels they are sheared: the viscosity expresses their resistance.

The melt index (MI) or melt flow index (MFI) is an inverse measure of viscosity. High MI implies low viscosity and low MI means high viscosity. Plastics are shear thinning, which means that their resistance to flow decreases as the shear rate increases. This is due to molecular alignments in the direction of flow and disentanglements.

Newtonian/non-Newtonian

Viscosity is usually understood to mean Newtonian viscosity in which case the ratio of shearing stress to the shearing strain is constant. In non-Newtonian behavior, typical of plastics, the ratio varies with the shearing stress. Such ratios are often called the apparent viscosities at the corresponding shearing stresses. Viscosity is measured in terms of flow in Pas (P) with water as the base standard (value of 1.0). The higher the number, the less flow.

Melt Index

The melt indexer (MI; extrusion plastometer) is the most widely used rheological device for examining and studying plastics (principally TPs) in many different fabricating processes. It is not a true viscometer in the sense that a reliable value of viscosity cannot be calculated from the measured flow index. However, the device does measure isothermal resistance to flow, using standard apparatus and test methods that are standard throughout the world. The standards used include ASTM D 1238 (U.S.A.), BS 2782-105°C (U.K.), DIN 53735 (Germany), JIS K72 IO (Japan), ISO RI 133/R292 (international), and others.

The standard apparatus is a ram type plasticator which at specified temperatures and pressure extrudes a plastic melt through the die exit opening. The standard procedure involves the determination of the amount of plastic extruded in 10 minutes. The flow rate, expressed in g/10 min., is reported. As the flow rate increases, viscosity decreases. Depending on the flow behavior, changes are made to standard conditions (die opening size, temperature, etc.) to obtain certain repeatable and meaningful data applicable to a specific processing operation. Table 1.6 lists typical MI ranges for the certain processes.

fable 1.6 Examples of melt index for different processes.

Process

Ml range

Injection Molding

5-100

Rotational Molding

5-20

Coating Extrusion

0.1-1

Film Extrusion

0.5-6

Profile extrusion

0.1-1

Blow molding

0.1-1

0 0

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