Technical Report: A Rheology Lesson

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Whether you are dealing with a fluid coating or a plastic product, you need to understand rheology. Rheology of the polymer in question affects not only the final product but the settings and limitations of the process.

But what is rheology? Stated simply, rheology is the “flow” of a polymer.

You have heard of viscosity and melt index, but these numbers provide only part of the answer. Rheology gives you an understanding not only of the polymer in question but how it interacts with the process. Since you most likely are not selling a polymer pellet, you are interested in how the plastic can form a sheet, film, or fill a mold best.

Understanding Shear

Why make the distinction between the study of polymer flow and the numbers people are used to — melt index, viscosity, etc.? Well, if you can understand what causes a polymer to move the way you want, flow in the direction you need, or fill the corners of your equipment properly, then you have accomplished both the product and process end of your manufacturing and converting operation.

So what happens inside the equipment that is making the polymer form the product you are making? The big concept to understand is shear. Shear is the force that deforms the polymer and is applied in the direction the polymer is flowing.

Take a peanut butter sandwich. When you open a jar of peanut butter, it will stay in the jar, even if you tip it upside down. However, if you move it around with a knife, the peanut butter moves easily. Spreading the peanut butter on the bread is an example of how some polymers are solid at room temperature but flow like a liquid when the proper force is applied.

This phenomenon of moving the polymer into a fluid with force is called shear thinning. Most polymers are like peanut butter; they become more rubbery the more they move.

Polymers also shear thin with heat. As the temperature goes up, viscosity goes down (and vice versa). So if you want to make a polymer become more fluid, apply shear force and heat. Now, if you are at a product or process limit, you also can mix in a less-viscous polymer to lower the point of heat and force needed to fluidize the polymer.

If you can point to a polymer that shear thins (like peanut butter), can you point to a polymer that shear thickens? If anyone has been sent a “walking on water” video, you have seen a shear thickening fluid in action.

In these home-brewed science experiments, cornstarch and water are mixed in a wading pool in the backyard. This new polymer is liquid at room temperature, but when a person runs quickly across the surface (therefore applying high shear force), the cornstarch mix thickens. Violà! A shear thickening polymer is formed. Feel free to try this experiment at home.

Viscosity Differences

Did I say we weren't going to talk about viscosity? Well, now that you know the difference between peanut butter and corn starch…I mean, shear thinning and shear thickening polymers…how much shear force is required to make peanut butter spread or cornstarch and water walkable? The answer lies in the understanding of viscosity. Viscosity is the consistency of the polymer at room temperature.

Did you know there are two viscosities of interest to a polymer processing operation? What we have talked about already — the resistance in the direction of flow — is shear viscosity, while the resistance to stretching is elongational viscosity. What's the difference, and why are the differences important?

Both can be understood if you imagine the polymer as a collection of rubber bands. As you freeze the rubber bands, they become stiff and unwilling to deform. These frozen rubber bands are similar to a polymer at room temperature. If you warm up the rubber bands to room temperature, they relax, and you can stretch them. However, if you stretch the rubber bands too much, they want to snap back to their original position.

What you are doing with all of the heating, cooling, curing, and forming in the equipment available to you is converting the polymer into a new shape where it is happy to stay. If not processed correctly, the polymer will want to move back to its original shape, causing defects like curl and warp.

Just keep the forces of heat and shear on the polymer until the product is in the best shape to settle. Heat and motion do wonders for polymers in process, especially if many expansions and contractions are required to form the final product.

Measuring Equipment

So now we understand how a polymer flows, forms a product, and resists change. How do you obtain this information about the polymer if it is so important?

You may have heard of a viscometer to measure viscosity. So, guess what? To measure rheology of a polymer, you use a rheometer.

The rheometer takes the polymer and presses it between two plates of a geometry that matches the range of application (flat, cone, etc.). At a series of process temperatures, these plates rotate at a set frequency. This vibrational movement and the resistance to the movement are measured.

Now you have the information you need to design equipment to push the polymer through the equipment and form a final product in the shape you need. Temperature, force, flow rate…they're all there!

Armed with the rheological information, an equipment manufacturer can provide you with the right solution for your process. If you need to form a thin product, the designer can look at the narrow openings that are required and provide the correct temperature points and expansions and contractions required to form the polymer with the least amount of stress.

Stress is especially important when you have a polymer that expresses both shear resistance and elongational resistance. As you can envision, if the polymer acts like a rubber band and is stretched many times the original length, it will want to snap back. An experienced equipment manufacturer will know, usually through trial and error, how to properly form the cavities the polymer will fill.

Multiple Layers

I love to order viscosity drinks when I go out. You know — the fancy cocktails in which the different liquors aren't mixed. Layers of liquid lay one on top of another and usually in different colors. These drinks are great, but if I mix them with a spoon or pour them into another cup, the turbulence will mix the liquids.

Now what if I made a “drink” of polymers of different viscosities? Luckily, hot flowing polymers typically do not experience turbulence when moving, so I can move my layered polymer drink from cup to cup and keep the layers separate.

The polymers don't mix because dissimilar polymers like themselves more than they like other polymers. This chemical incompatibility, in association with the viscosity differences in the materials, makes laminar flow (versus turbulent flow) possible.

Whether you need to form single or multiple polymer layers, the understanding of the rheological character of the polymer is critical in the equipment manufacturing, material processing, and final product quality of your operation.

Mark Miller, author of PFFC's “Coating Matters” column, is an independent consultant in the roll-to-roll coating industry and owner of Coating Tech Service, Eau Claire, WI. As a certified Six Sigma Black Belt for 3M, Mark has been integral to new developments and technology in web coating applications. He holds a Master's degree in Polymer Science and Engineering from Lehigh Univ. and a Bachelor's degree in Chemical Engineering from the Univ. of Wisconsin. Reach him at mark@coatingtechservice.com or 715-456-9545.

The views and opinions expressed in Technical Reports are those of the author(s), not those of the editors of PFFC. Please address comments to the author(s).


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