In order to understand how a plastics behaves during processing we need to know how the plastics melt flows. One of the most important flow properties is the viscosity of a fluid. Viscosity is the resistance to flow. A low viscosity fluid, such as water, flows easily. A high viscosity fluid, such as honey, flows less easily [slide]. The viscosity in shearing flow is the shear stress acting on the fluid divided by the shear rate [slide], this will be explained later.
Since plastics are made up of very long molecules, they have high melt viscosities. The viscosity is often expressed in the units Pascal.second; the units Poise are also used. It is easy to convert from Poise to Pa.sec: ten Poise equals one Pa.sec. This table shows the approximate viscosity of various fluids, expressed in Pa.sec.
MATERIAL VISCOSITY IN PASCAL.SECOND
AIR 0.00001
WATER 0.001
OLIVE OIL 0.1
PLASTICS MELTS 100 to 1,000,000
PITCH 1,000,000,000
It is clear from the table that the viscosity of plastics is much higher than the viscosity of water, by at least five orders of magnitude! With a higher plastics viscosity, more torque is required on the extruder and more pressure is necessary to force the plastics melt through the die. The viscosity of a plastics is strongly dependent on the molecular weight of the plastics; the higher the molecular weight, the higher the viscosity. Since for one plastics, for example polyethylene (PE), there are many grades with different molecular weights, the viscosities can vary substantially from one polyethylene to another.
Melt Index
The ability of a plastics melt to flow is often measured in a melt index tester. The melt index machine is a simple ram extruder.
Plastics is placed in the reservoir and heated to the appropriate temperature. A weight is placed on top of the ram and this causes the plastics melt to be extruded out of the melt index die located at the bottom of the reservoir. The melt index (MI), sometimes called the melt flow index (MFI), is the amount of plastics extruded in grams in a certain time period, usually ten minutes.
A low viscosity plastics will flow out faster than a high viscosity plastics. Thus, a high MI is indicates a low viscosity plastics and a low MI a high viscosity plastics. The term fractional melt plastics is often used; this means that the plastics has a melt index less than one. A melt index less than one is considered low and, thus, fractional melt plastics have high viscosity.
The Effects of Shearing
When a is processed it is usually exposed to shearing flow. This is due to different layers of the plastics moving at different velocities. The rate of shearing that occurs in a fluid is called the shear rate; it is the difference in velocity between two fluid elements divided by the normal distance between the elements.
The shear rate is determined by the flow rate and the geometry of the flow channel. When the flow rate of the plastics is high, the shear rates will be high. Also, when the flow channel is small, the shear rate will be high.
Shear Thinning or Pseudoplastics Behaviour
In plastics, the viscosity changes when the shear rate change. A fluid that behaves that way is called a non-Newtonian or non-linear fluid.
The viscosity of plastics melts reduces with increasing shear rate; this is called shear thinning or pseudoplastics behaviour. This behaviour is due to the fact that the plastics molecules are very long and entangled. The entanglements of the molecules determine the viscosity of a plastics. When a plastics is exposed to a high shear rate, the number of entanglements of the molecules reduce and with it the viscosity.
When the shear rate reduces, the viscosity increases again. This behaviour is called shear thinning behaviour, it is also called pseudoplastics behaviour. This behaviour is very important in extrusion.
If we plot the viscosity vs. shear rate for a fluid and it forms a straight line on a log-log plot, we call the fluid a power law fluid [slide] , see figure 4 (vis_srt1.cvs). If the actual viscosity-shear rate curve is close to a straight line, the actual behaviour can be approximated with a power law expression.
The power law equation has two important parameters: the consistency index and the power law index. The consistency index is the value of the viscosity at a shear rate of one. The power law index is a measure of the degree of shear thinning behaviour; for plastics it varies between zero and one. The closer the power law index is to zero, the more strongly shear thinning the plastics. When the power law index is close to one, the plastics is only slightly shear thinning. When the power law index equals one the viscosity is not affected by shear rate; a fluid that behaves this way is called Newtonian.
Effect of Temperature on Viscosity
When the temperature of a plastics melt is increased the viscosity reduces.
The effect of temperature on viscosity varies from one plastics to another. In general, amorphous plastics have a high temperature sensitivity relative to semi-crystalline plastics.
The temperature coefficient for amorphous plastics ranges from about 5 to 20 percent. This means that the viscosity changes from 5 to 20 percent for each degree Centigrade change in temperature.
For semi-crystalline plastics the temperature coefficient of the viscosity is about 2 to 3 percent. A change in the extruder barrel is going to have a larger effect on an amorphous plastics than on a semi-crystalline plastics. Good temperature control, therefore, is even more critical in amorphous plastics than in semi-crystalline plastics.
Viscous Heat Generation
When a plastics melt is sheared, heat is being generated in the plastics; this is called viscous heat generation. The viscous heat generation is determined by the product of viscosity and shear rate squared. Thus, the higher the viscosity of the plastics, the higher the viscous heat generation. The same is true for the shear rate, however, the shear rate has a stronger effect since the viscous heating increases with the shear rate squared.
As a result of the high viscosity of plastics, in extrusion most of the heating of the plastics comes from viscous heat generation. In fact, in some cases too much viscous heat generation occurs in the extruder and the machine has to be cooled to maintain the desired melt temperatures.
