Understanding the Incumbent Resin Effect

New and innovative products are constantly produced on pilot and manufacturing lines using a variety of processes, including PE blown and cast films using single-screw extruders. Many times, these films will be sent to end users for evaluation. Acceptable quality and properties of the films are key to the success of the new product. Most of the time, the new film produced has acceptable quality (very low gels) and great physical properties. These new products are produced by replacing the incumbent resin with a new resin or challenger resin. Often, the new resin decreases the cost of the product.

The incumbent resin effect starts off by running a single-screw extruder for extended periods of time with the same resin (incumbent) during typical production. Here, the extrudate and film appear acceptable with only a few gels and black specks in the film product. These gels and black specks are generated in stagnant regions of the screw. Most of the degraded resin, however, is attached to the screw and is stable. That is, not separating from the screw and appearing in the film product at a rate high enough to alarm the quality specialists in the plant.

Next, as a short trial or prototype run, the incumbent resin is switched with a challenger resin. Even though the challenger resin may be very similar, it will likely process slightly differently than the incumbent resin. This slight difference in processing is often enough to cause the old and degraded material that is adhering to the screw to separate from the screw and contaminate the extrudate. The old degradation will start to come out of the die, typically in about five minutes after the switch for blown film lines. The initial discharge of gels is sometimes viewed as a gel shower, and then a high level of gels will continue to be observed for the short duration of the trial run.

In many cases, plant personnel will unknowingly blame the high level of gels on the last change — in this case, the switch to the challenger resin. That is, the challenger resin is incorrectly blamed for the high level of gels. In severe cases, the trial is stopped and the challenger resin is eliminated as an acceptable resin for the application. The incumbent resin continues as the preferred resin. The root cause for the gels, however, is a poorly designed extruder screw and not the challenger resin.

A necessary condition for the incumbent resin effect is minor design flaws on the screw. These flaws are regions where small amounts of resin can stagnate, degrade and then separate from the screw, causing defects in the film product. The level of gels is manageable at steady-state conditions for the incumbent resin. However, the slight upsets that occur by introducing a challenger resin can cause the degraded material to separate from the screw at a faster rate. If the challenger resin was processed for an extended period, likely the same level of gels would eventually occur as that for the incumbent resin. Typically, the challenger resin is extruded for only short trials and is incorrectly blamed for the higher level of gels.

Most screws designed and operating in North America for PE resins are single-flighted designs with a barrier melting section and a downstream Maddock style mixer. The most common flaws include flight radii in the metering channels that are too small and improperly designed flutes on Maddock mixers. If the design of the screw is proper, the incumbent resin effect will not occur.

The most common defect on screws built for PE resins is the size of the flight radii. Most manufacturers design screws with flight radii that are about half of the depth of the channel. In many cases, the size of the flight radii is even smaller. Figure 1 shows the flight radius for an improperly designed screw. Channels with small radii can cause regions where the residence time is extremely long, leading to resin degradation due to the formation of secondary recirculation flows known as Moffatt eddies. Photographs of resin degradation due to small flight radii are shown in Figure 2. Flight radii that are large at about 1.5 times the local channel depth will not allow Moffatt eddies to form, eliminating degradation at this location.

Maddock-style mixers are designed into most screws for PE extrusion processes. Their widespread use is due to their low cost to build, simplicity of the design, and their ability to trap, melt and disperse solid polymer fragments from incomplete melting. Poorly designed Maddock mixers, however, can cause resin to stagnate and degrade. Proper design of the devices was discussed in my March 2024 column.

 FIG 2 Degraded resin due to small flight radii because of Moffatt eddies.

A common design flaw is to position too many flute pairs on the device, creating the need to cut these flutes very deep such that pressure consumption is acceptable. The deep flutes can cause resin to degrade on the sides of the channels, as shown in Figure 3. Like the degradation shown in Figure 2, the degradation in the Maddock mixer will be released from the screw slowly for the incumbent resin but will release at a higher rate for the challenger resin due to differences in rheology and processing stability.

As previously stated, the degraded resin in the extrudate is typically first observed within one residence time after switching to the challenger resin or typically less than five minutes for film processes. If the challenger resin was the source for the degraded resin (gels) then the gels either had to come from the resin manufacturing plant or the challenger resin would need to degrade in five minutes or less to a hard carbonaceous material. However, much longer times are required to degrade PE resins.

A necessary condition for the incumbent resin effect is minor design flaws on the screw.

Modern resin manufacturing processes exclude oxygen from the system and are very streamline such that process areas with long residence times do not exist. As such, crosslinked and oxidative gels are likely not generated by the manufacturer. Moreover, modern quality control techniques are typically in place to monitor gels, preventing off-specification resin from leaving the plant as prime.

Other sections of the process can also contribute to a higher rate of degraded material in the product for a challenger resin. These include transfer lines that are deigned too large in diameter, and dies and screen packs that are not streamlined. All can contain regions where a low shear stress at the metal wall can occur, creating regions that can cause resin to stagnate and degrade.

FIG 3 Maddock mixer with flutes that are too deep cause resin degradation at the pushing and trailing sides of the flutes.

There are two technical solutions to mitigate the incumbent resin effect for PEs. The first and long-term solution is to design a new screw that does not contain regions where the resin flow will stagnate. This means the flight radii must be large enough in the liquid-filled sections of the screw to eliminate Moffatt eddies, and mixers must be streamlined such that stagnant regions do not exist. Designing and building a new screw will take six weeks or more for delivery. The new and properly designed screw will enable a nearly gel-free product.

The second and short-term technical solution is to remove and clean the existing screw before the challenger resin is introduced to the extruder. For this solution, the extruder may operate without producing degradation products for several hours. After this induction period, the degradation products that are formed may be stable and attached to the screw, causing only a low and manageable level of gels. This gel level would be essentially equivalent to the level produced by the incumbent resin.

About the Author: Mark A. Spalding is a fellow in Packaging & Specialty Plastics and Hydrocarbons R&D at Dow Inc. in Midland, Michigan. During his 37 years at Dow, he has focused on development, design and troubleshooting of polymer processes, especially in single-screw extrusion. He co-authored Analyzing and Troubleshooting Single-Screw Extruders with Gregory Campbell. Contact: 989-636-9849; maspalding@dow.com; dow.com.