Blown Film Extrusion (eBook)

An effective way to understand polymer melt flow is to contrast the behavior of an ideal solid under an applied force with the behavior of an ideal liquid. Figure 2.1 shows an ideal solid undergoing an instantaneous, constant stress (applied force) for some time until the stress is removed. Notice that upon application of the stress, the solid strains (deforms) instantly and then does not strain any more until the stress is removed, at which time it fully recovers to zero strain. This is the action of a typical spring and is known as elastic behavior.

Figure 2.2 shows the different behavior of an ideal liquid undergoing the same instantaneous, constant stress. The liquid begins to strain upon application of the stress and continues to strain as long as the stress is applied. When the stress is removed, the liquid no longer strains, but also does not recover. This permanent deformation, or flow, is known as viscous behavior. For polymers, we can say this occurs when chains slide past one another.

In extrusion, we consider two means of applying stress to the polymer melt, that is, two driving mechanisms for flow: pressure and drag. Pressure flow occurs when the melt is in a channel, such as inside a die, where there is a higher pressure at one end of the channel than at the other. The pressure of the melt entering a die from an extruder is higher than the pressure at the outlet of the die, where it is zero. As a result, the difference in pressure drives the melt through and out of the die.

Drag flow occurs when the melt is in a channel, such as between the screw and barrel, where one surface is moving relative to the other. The friction between the melt and each surface as well as the relative motion between the two surfaces causes the melt to flow. This is the primary driving mechanism for moving melt through an extruder.

As the melt is flowing through an extruder or die, there are two primary strain fields that it can experience: shear and elongation. Shear flow is defined as two adjacent flow streams traveling at different velocities. Figure 2.3 compares a fluid element at rest to one in shear. Notice that in shear, the top of the element is flowing faster than the bottom of the element.

Whether melt is moving through a die due to pressure or through an extruder due to drag, the melt is experiencing shear because of the presence of friction with the channel walls. In other words, some regions of melt in the channel are flowing faster than others. Shear occurs anywhere melt flows through channels. In fact, the degree of shear imparted on the melt varies through different size channels in an extruder and has a profound influence on how the melt flows, as discussed later in this chapter.

Elongation is defined as the acceleration of melt along a flow stream. It is also called extension or stretching flow. Figure 2.4 compares a fluid element at rest to one in elongation. Notice that in elongation the increasing speed along the streamline causes the element to be stretched in the direction of elongation. This occurs most obviously as melt is emerging from a die and is being pulled by the haul-off equipment.

Elongational flow plays a very important role in blown film extrusion. As melt emerges from a blown film die, it is being stretched in two directions at the same time. The nip rollers are pulling the melt faster than the speed at which it is emerging from the die, hence stretching it in the machine direction. At the same time, the internal air pressure is expanding the bubble to a diameter greater than the die diameter, hence stretching the melt in the transverse direction. These deformations are taking place up to the frost line, where the melt solidifies, and are imparting vital solid-state properties to the film as discussed in Chapter 5.

As implied above, both shear and elongational flow are driven by stress (applied force) and result in strain (deformation). It is valuable to be able to quantify the amount of stress in either case (shear stress or elongational stress). Likewise, the amount of strain is important to quantify, but this is generally done as a rate of strain (shear rate or elongational rate). The equations and methods involved in these analyses are beyond the scope of this book, so the interested reader is referred to textbooks on rheology [6–9]. However, the concepts of shear or elongational stress and shear or elongational rate will be referenced further in this chapter.

An interesting phenomenon that occurs in polymer melts during flow is that the chain-like molecules tend to orient in the direction of flow. Figure 2.5 shows a schematic of the influence of shear rate on molecular orientation. At rest or in very low shear, chains are randomly coiled with no particular orientation. However, as the shear rate in the melt increases, such as at higher extruder screw speeds or through smaller die gaps, the chains become more oriented in the direction of flow. This occurs similarly with elongational flow.

So far, the driving mechanisms for flow (pressure and drag) and the velocity fields established during flow (shear and elongation) have been discussed. Another important variable is the polymer melt’s internal ability to resist flow. This is called viscosity. Melts that are highly resistant to flow have high viscosity and melts that flow very easily (i.e., with little force) have low viscosity. (Incidentally, when the term viscosity is used, this usually means shear viscosity as opposed to elongational viscosity.)

In general, polymer melts have very high viscosity compared to other liquids. A polyethylene melt may be one million times more viscous than water. The reason for high polymer melt viscosity is the entanglements that occur between chain segments at the molecular level. The longer the chains (i.e., the higher the molecular weight), the more entangled and, therefore, the more viscous the melt.

As mentioned above, the flow of polymer melt is somewhat complex. Viscosity is not simply a constant value for any given polymer. In fact, the viscosity of a melt varies as a function of several conditions.

This section describes the important extrusion variables that influence melt viscosity. It is divided into two sections: process conditions and molecular structure.