New Screw Boosts Mixing, Enhances Polymer Properties

Undaunted by commonly held beliefs that “there is nothing more that can be done to improve single-screw extrusion,” and forever the tinkerer, Keith Luker has for the last two decades been working on the next iteration of his SFEM Elongational Mixer. What the founder and president of Randcastle Extrusion has perhaps stumbled onto is a novel device that is not only a more effective mixer but, in some cases, improves material physical properties and makes pre-drying unnecessary.

Luker has dubbed it the Molecular Homogenizer (MH). It’s a new device that he says could have broad appeal across the material supply chain — beginning with resin manufacturers. It’s so named, he maintains, because it mixes material at the “molecular level.” He elaborates, “The mixing is so fine that the properties of polymers change.” “Physical properties of polymers (virgin polymers processed through the screw) improve. In hygroscopic polymers, water-vapor absorption is postponed. Undried hygroscopic polymers show no bubbles. Rheological properties show incredible improvement.”

Luker has presented papers on this mixing device in 2022 and 2023 at Plastics Technology’s Extrusion Conference, and more recently this past June at the SPE Extrusion Division Screw Design TopCon. Luker is the first to admit that more work, testing and research needs to be done to adequately explain why the design is performing as it is. That said, he has patents pending in five countries and Europe, and had the MH tested on a wide range of materials, including PLA, PMMA, PET, PET with 25% reclaim, PEEK, PVA, PVA with 3% reactive agent, PLA, SAN color concentrate, PC, nylon and coffee chaff in LDPE. The mixer is also being used by at least one extrusion processor in a production environment.

The tested screw had at least 100⁷ (100 trillion) mixing actions along the XYZ dimensions. “Such an increase resulted in mixing to the small molecule level (such as 3 atom water vapor),” he says. “Because the sequence is orderly, the mixer imposes organization to the messy, tangled mixture produced by a polymer reactor. We believe this mixing changes the alignment of long polymer chains; distributes small molecules (monomer, water vapor), additives, particulate, actives, etc. This results in the improvements we see at the macroscopic level. The mixing may enhance, indirectly, diffusion.”

Mixing Forces

Luker notes that many conventional mixers rely on shear with compression forces to push material through narrow gaps. Compression forces, he contends, are negative and counter mixing. That is, when compression is combined with shear or elongation, mixing performance is significantly impaired.

Luker elaborates: “When compression is applied equally and opposite in all three dimensions, it is called pressure. When we apply compression or pressure to a cube, sphere or bubble, mixing does not occur. The force is applied in all three dimensions inwardly. If the cube or sphere were made of loosely held particles, then compression or pressure will push these particles together causing agglomeration. When pressure is applied to the bubble, it shrinks, becoming denser and this too is agglomeration. Agglomeration is the opposite of mixing and is caused by compression and pressure. Think of pressure as an antimixing force.

On the other hand, reorienting or interrupting shear makes “order of magnitude improvements” possible, he adds. Further, when a mixing flow is oriented in one direction and then another, “The orientation imparted by the first section is destroyed, and each ‘stage’ of the device behaves as a separate unit.”

In 2005, Luker developed the SFEM mixer that created significant elongational mixing. Dow Chemical presented a paper at ANTEC 2011 which showed mixing results that were eight times better than their twin with immiscible polymer blends. This new mixer uses first principal arrangements of interrupted shear, interrupted elongation and a mixing flow called inversive mixing.

“Until this arrangement, it was not understood that compression was so detrimental to extrusion mixing,” Luker says. “Stated another way, if there were no compression mixed in with shear and elongation, they would mix vastly better. Fundamentally, this is why the MH produces such surprising results.”

The design arrangement, Luker maintains, “creates exponentially improved mixing such that surprising results start to become mundane.” For example, processing through the unvented Molecular Homogenizer has been shown to:

• Improve Physical Properties: Increase tensile-at-yield greater than the virgin polymer.
• Increase Viscosity: Restore undried hygroscopic regrind to the viscosity of the unprocessed virgin polymer.
• Slow Water Absorption: Processing slowed water absorption from the typical 4 to 6 hours when exposed to atmosphere to over 72 hours
• Sequester Water Vapor: Moisture analyzer measurements showed the moisture content of an unprocessed material entering the Molecular Homogenizer at 0.328%; it then reported that the Molecular Homogenizer processed polymer had 0.171% moisture.

How It Works

Randcastle Molecular Homogenizer has a 36:1 L/D and features seven mixing sections. According to Luker, this design promotes a powerful dispersive mixing force called trilongation, where all the flow is stretched in three dimensions. Each of the mixing elements of the device creates a repetitive mixing sequence twice: 3D trilongation, 2D elongation and 1D shear. Each mixer is known to improve the mixing by more than 100 times, Luker says.

He says that unlike a twin-screw extruder, the entire flow moves through this sequence. Each time it does, the 100-fold improvement is created, a bit like a static mixer multiplication except this is dynamic. He says this creates more than 100 trillion mixing events in the screw. At the end of each mixing element, the flow is inverted where the inside becomes the outside. This
reorientation enables the friction-warmed material at the screw to move to the barrel where cooling takes place before the start of the next mixing element. In degassing applications, it moves the trapped bubbles to the extensive surface for rupture and venting.

Test Results

Undried hygroscopic polymers are known to produce visible bubbles in unvented extruders. Luker reports that in his lab the new mixing screw processed PMMA, PET, PET with 25% amorphous reclaim, PEEK, PVA, PVA with 3% reactive agent, PLA, SAN color concentrate (black and white), PC and coffee chaff in LDPE. None of these materials were predried, and none showed any evidence of bubbles (water). More specifically:

• PET With 25% Reclaim. Undried PET is very well known to absorb water vapor and form bubbles. Undried PET reclaim is amorphous and known to absorb even more water. No bubbles were seen.
• Reactive Extrusion of PVA: Undried PVA produces a bubbled extrudate. A reactive chemical was added to dried PVA and created a bubbled extrudate in a conventional screw — implying that the bubbles were not water vapor. Yet, in an undried PVA with 3% reactive agent, no bubbles were seen in the Molecular Homogenizer extrudate.
• Coffee Chaff: Coffee chaff is the thin papery skin that comes off the coffee bean. When heated with LDPE, the chaff breaks down, releasing gases such as carbon dioxide (CO₂), water vapor (H₂O), carbon monoxide (CO), and various volatile organic compounds (VOCs). The Molecular Homogenizer produced no bubbles in the extrudate.
• Black and White SAN Color Concentrate: SAN is hygroscopic. Both carbon black and TiO₂ are hydrophilic and also absorb water vapor. The undried materials were both pelletized. No bubbles were visible in the strand cut pellets. The pellets were then processed in a conventional screw on a film line.
• Nylon: Nylon is a very hygroscopic polymer absorbing moisture in the 2%-9% range. It is expensive to dry polymers. In the Randcastle lab, Luker processed a nylon blend into film on a conventional screw for a week. Drying was necessary. Conditions were well established.

He then swapped out the screw with the Molecular Homogenizer and processed the same nylon but didn’t dry it first. Bubbles were still created. He then opened a single atmospheric vent, exposing the extensive free surfaces, and the film on the right was produced. He notes the output rate was the same (rpm was increased and extruder starve fed); the temperatures were kept the same as the dried material; no die adjustments were necessary to produce the same tolerance film — implying the viscosity was the same; and pressure was the same as the dried material — implying the viscosity was the same.

• Undried PVA With More Than 3% Reactive Agent: PVA with 3% reactive agent was one of the materials where the strand did not show bubbles. However, the experiment also mixed 6, 9 and 12% reactive agent. At 6%, bubbles were created. A single atmospheric vent was opened. No bubbles were seen. The experiment proceeded and no bubbles were seen at 9 or 12%.

Luker reports the Molecular Homogenizer has been shown to slow the rate of moisture absorption, which he notes is particularly useful for regrind where it takes more than a couple of hours to grind and reprocess the material. For example, PMMA regrind will absorb moisture in 4 to 6 hours, creating problems (bubbles, lower viscosity, property degradation and roughened surfaces), he points out.

He recalls that the screw was used to process dried PMMA through a water trough and strand pelletizer on a rainy, summer Friday afternoon in New Jersey. The bag of pelletized material was left exposed to the atmosphere over the weekend, and it continued to rain. On Monday afternoon, the pellets were extruded (undried) in a conventional screw. There were no bubbles. This, Luker says, demonstrates that the time for processing regrind can be substantially extended for product improvement.

In yet another test, a Molecular Homogenizer and a general-purpose control screw were used to process virgin dried PETG. The pellets were then molded in a family mold and compared at the Pennsylvania College of Technology. Concerning properties of test samples, says Luker:

• Elongation at Yield: The Molecular Homogenizer was 17.9% better than the general-purpose screw and improved this property 6.7% vs. the virgin material.

• Izod Impact Strength: The new mixing screw was 22.4% better than the general-purpose screw. The Molecular Homogenizer screw was 18% worse than the virgin pellets.

Who Would Benefit?

Luker states many applications that would benefit from the use of this new mixing screw technology. He notes, “Start with polymer producers. The MH has demonstrated postreactor improvement in physical properties and rheology — despite an additional processing history. Since the reactor operation already includes a pelletizing processing history, we expect the same or better results from a Molecular Homogenizer placed at the end of a polymer reactor for pelletizing. This will create polymer with better properties and lessened or no need for drying.”

For compounders and those involved in direct extrusion (bypassing the pelletizing step), Luker says the MH serves as a stable, high-pressure pump that eliminates the need for a gear pump. In reclaim operations, meantime, he asserts that the lower water absorption rates demonstrated translate into increased time for processing without the need for drying. Viscosity improvements with reclaim also mean more consistent die flow, he adds. Higher quality and improved viscosity often lead to increased output, Luker notes.

Other applications that Luker believes to be well suited for the Molecular Homogenizer include: processing of PP, PFA and rigid PVC powder, which he says have been shown to run at higher rates with the new screw; research and development, as greater mixing performance could pave the way for new material combinations; and users of twin-screw extruders, where the screw can be used as an add-on.