Go with the Flow: Rheology, Polymer Processing, & Materials Selection

viscosity-test-machineAre you a naturally relaxed, easy going kind of person? One might even say that you seem to “go with the flow”? In life, there are these types of individuals, just like there are those that are unyielding, stubborn, and often referred to as “Type A.” Interestingly enough, polymers and plastics can be characterized by their ability to “go with the flow” also.

We might not refer to plastics and polymers as type A or stubborn, but there are adjectives used to characterize and explain the behavior of these materials. In a very simplified definition, viscosity is the resistance to flow.

Why does viscosity matter for materials? In general terms, polymers are injection molded, compression molded, or extruded– all of which require forcing a polymer into a shape. Therefore, viscosity is especially important for polymer processing. If a material is more resistant to flow and you must make it flow anyway, you have to provide more force and adjust other conditions, like temperature. Manufacturers must carefully understand this information to ensure proper conditions and best use of resources.

Like much of life, it isn’t “that” simple. You should not select a polymer with a low melt viscosity, for ease of processing. The reason a polymer is easier to process, a lower molecular weight, may cause the product to have inferior properties. When properly formulated and produced, manufacturers consider the “cradle to grave” lifecycle of plastic products; the source of the raw material, the sustainability of the supply chain, the ability to process the polymer at the required rate, and yield products that serve in the intended way for the expected duration.   

Some products have a useful lifetime of a few minutes (plastic bags and water bottles for example) whereas other products have expected lifetimes of 50 years or more (insulation on wiring and vinyl siding.)  In all aspects of polymer science, there is not usually a “good” or a “bad” polymer because there are so many factors that need to be considered and balanced in order to determine the best material selection for a particular set of circumstances.

Considering the full context of the polymer product is a must, in order to make wise materials selections. Things often go wrong when only one factor is the driving force for a decision–like only considering cost or only considering aesthetics. Consider this: If the raw polymer material has to be the lowest cost, no matter what, it may not be able to make products that satisfy customer expectations. Similarly, if a good polymer is purchased but the production rate is forced to be too high, it may lead to increased overhead costs to produce and therefore the higher price point for the consumer.

When working with polymer processors and manufacturers to help them understand a material’s performance and characteristics for processing, we use rheology. This analytical technique is the “go to” for determining the response of plastics to shearing and viscosity response over time. Not all plastics are suitable for this analysis, though. It is used for thermoplastics, which are plastics that melt when exposed to heat. Thermosets are not suitable for rheology because they do not melt due to their cross-linked structure. Rather than melting, they simply degrade.

Thanks to viscosity, materials can “go with the flow” or be more stubborn. If you find yourself perplexed by how your plastic is performing, go ahead and reach out to us.