- October 02, 2012, Mark Miller
The best starting point for polymer understanding is rheology, or the flow of the polymer in fluid form.
What is rheology?
ρηεολογψ—It’s all Greek to me! Yes it is. But it doesn’t need to be. The term rheology comes from the Greek word rheos, which means “to flow.” Rheology is the study of flow. When you talk about squeezing, spreading, or lubricating a fluid, you are talking rheology. When you apply a force that causes a fluid to move, rupture, or flow, you are describing a rheological force.
In plastics, everything boils down to mayonnaise and Silly Putty. While Silly Putty is more fun to play with, let’s examine mayonnaise first, because most plastics in the melt phase behave just like this tasty sandwich spread. Both plastics and mayo are solid at room temperature and liquid when broken by a shear force, which is a force applied in the same direction as the polymer is flowing. When you apply mayo to a sandwich with a bread knife, you are generating shear force. A common example in plastics is the force applied by the flights of an extruder screw as polymer moves through the barrel.
Like mayo, most polymers are shear-thinning materials. The faster they move, the softer (less viscous) they become. The heat in your equipment system helps each polymer you process get to the right consistency before this shear force is applied. Heat makes the polymer molecules move around more, allowing them to be sheared more readily. Viscosity decreases as temperature increases. Another way to lower viscosity is to mix a lower viscosity material into a higher viscosity material. The shear force will now have a low viscosity plane to push on, lowering the overall viscosity of the mix. High molecular weight materials are larger and more difficult to shear. This is also true of highly branched polymers. The bigger the polymer particle, the more force it will require to move it into the shape we want.
While mayo shear-thins, Silly Putty shear-thickens. It is soft and pliable at low shear rates (squeezing it with your hand), but have you ever hit Silly Putty with a hammer? It shatters! If you pick up the pieces, however, they are still soft and pliable and will actually combine into one blob.
What is Viscosity?
We’ve established that viscosity is important as to how a polymer is going to flow in a given process, but what is it exactly? Viscosity is resistance to physical change or deformation. You also could refer to viscosity as the consistency of the material. Shear viscosity is resistance to deformation between layers of a polymer, while elongational viscosity is resistance to deformation from stretching the polymer.
Picture the polymers that make up a certain volume of plastic as a clump of coils or springs. The cooler the polymers, the tighter the springs are. The warmer the polymers, the more relaxed the springs become. Upon cooling, the polymer would like to “spring back” into a tight set of coils (this can result in edge bead). The trick is to “teach” the polymer a new position of rest (as a sheet instead of a pellet). Once the polymer is in motion, the best thing is to keep it in motion until you have the final shape you want. Polymers move at a speed and in a direction controlled by the force you apply. So, by a series of contractions, expansions, and diminishing patterns in polymer-forming equipment, the polymer responds to form the sheet, film, or product shape we need.
Key to Optimum Equipment Design: Rheology
The viscosity of a polymer characterizes that material and tells the equipment designer what to expect when it comes to flow and pressures throughout the system. To measure viscosity, a rheometer is utilized. Typically the pressure, flow rate, and fixed geometry of the rheometer determine the calculated viscosity. The calculated viscosity provides the basis for the design of the polymer equipment.
As you can imagine, understanding how a polymer flows in and around surfaces can be critical when you are designing equipment to transform molten polymer into a useful product. One key to gaining this understanding is to study the shear viscosity of the polymer melt at the processing temperature utilized in your production line. This is because shear viscosity is the resistance to flow between layers of a polymer (think of a stack of playing cards).
Another key rheological parameter is elongational viscosity—the resistance of a polymer to stretching (think of a rubber band or bed spring). Viscoelastic materials are materials in which we have to be concerned with both the (shear) viscosity and the elasticity (elongational viscosity). Materials such as polystyrene and PVC, which exhibit elastic characteristics, also resist stretching. Equipment designers need to allow the stretchy polymer to fill the shape desired more gradually. Otherwise, the “elastic memory” of these polymers prevents the shape from holding the way we would like.
Rheology Is Also Critical in Coextrusion
I was told once that anything can be described in terms of beer. I have found this to be true, even in discussing rheology. Unlike polymers, which are shear-thinning materials, beer does not shear-thin. If we double the head (pressure) as beer goes through a tap (pipe), the flow rate doubles; if we double the pressure on polymers, however, the flow rate more than doubles, because of shear thinning.
Beer flows like water and exhibits turbulence effects, such as mixing. Molten polymers, in contrast, exhibit laminar flow—flow that is smooth and streamlined. Laminar flow makes coextrusion possible. While the different layer materials in a turbulent fluid would intermingle, molten polymers exhibit smooth, streamlined flow.
Why don’t polymers mix easily? Besides chemical incompatibility, the viscous forces that are high in polymers and low in beer push dissimilar materials apart. Layer interface stability is controlled by shear stress applied to the polymers and viscosity inherent in the polymers. This is more problematic as skin layers become thinner. If the core material is the thickest and has the highest elongational viscosity, the flow is more stable.
While analyzing the flow properties of polymers is especially important for coextrusion, the fact is that rheological data are critical in any equipment design. Rheological information benefits the equipment designer and the plastic consumer by providing a fundamental understanding of how a polymer flows and allowing for more efficient, accurate, and reliable performance.
Roll-to-roll industry expert Mark Miller, owner of Coating Tech Service, has 14+ years of slo die coating experience and troubleshooting. Contact him at 715-456-9545; firstname.lastname@example.org; www.coatingtechservice.com.