Many new international players have emerged in the biopolymer field, using new materials and many new formulations. Co-Rotating twin screw extruders offer excellent material blending capabilities for all polymers, including biopolymers. Regardless of the formulation, you should consider your extruder equipment supplier as a development partner to achieve your goals in terms of melt and extrusion quality, productivity, and other parameters. Make sure your extruder equipment supplier has the facilities and expertise to help you test your material formulations and improve your processes.
As with any extrusion project, a degree of iterative experimentation is inevitable in the process development of a new biopolymer formulation. Some are successful from the outset; others require more work before they can be commercialized.
We have broadly classified biopolymer extrusion compounds into three categories.
- Starch-based polymer extrusion compounds.
- Bio resin extrusion compounds containing resins such as PHA, PLA, or PHB.
- (Biopolymer composite mixtures, defined as compounds containing two or more resins, starch-based polymers, and/or petroleum-based polymers).
Global bioplastics extruder capacity is expected to increase to 4.8 billion pounds by 2024, thanks to extrusion equipment manufacturers and brand owners steadily turning their attention to these "green" materials. If you're looking to supply extruder compounds to meet this demand, here are some extruder machine key processing considerations you need to know.
There are some extruders that are designed with extreme flexibility in mind. You can purchase long extruders with 48:1 or 52:1 L/D, and they have multiple exhaust port and feed position options.
Because biodegradable plastics are a rapidly evolving industry, there are often new ideas, new or improved feedstock materials, additives, etc. It is much easier to open or close available exhaust positions, or to add or remove extruders side feeders from available positions, rather than purchasing extruders new barrel tubes or expanding the extruder after the initial capital purchase.
Plan for at least one extruder;atmospheric vent (sometimes two) and at least one extruder vacuum vent (sometimes two). Biopolymers are usually hygroscopic and need to be handled with care prior to processing. Placing a vent early in the extrusion process is effective to minimize hydrolysis of the biopolymer and to avoid pre-drying the material.
Extruder vacuum venting is required. Do not consider the extruder vacuum system as a non-essential add-on; it is an integral part of the extrusion process and is often the difference between making a quality product and not. A well-designed plastic extruder vacuum system also addresses the often corrosive nature of biopolymer volatiles, which can pay for itself through reduced maintenance and downtime and increased production efficiency.
Biopolymers degrade rapidly when exposed to excessive heat and/or shear. The shear exposure of the material is proportional to the extruder screw speed and inversely proportional to the clearance between the extruder screw and barrel. For a given throughput rate, run the extruder at the lowest possible screw speed until the extruder torque is too high (>90%) or a volume limit is reached, which is defined by the compound support feed port or vent.
Volume limitation can be avoided by strategically placing the extruder vent to allow air and moisture to escape, and by placing the limiting kneading block or reversing screw element as far downstream of the feed or vent as possible. High torque can be avoided by designing the extruder screw to have a long, "gentle" mixing zone rather than a short, vigorous mixing zone. Stabilize the extruder feeder to minimize torque fluctuations, which allows the operator to run the extruder at a higher average torque.
The co-rotating twin-screw extruder, while an effective and efficient mixing device, is only 8 to 15 percent efficient as a pump, which means that high extruder outlet pressures can lead to significantly higher melt temperatures. High outlet pressures can also lead to exhaust streams from the exhaust port near the extruder outlet.
Increasing the diameter of the die head hole, increasing the mesh size of the strainer, and in the case of underwater pelletizing, increasing the water and die head temperature, all minimize the extruder outlet pressure. A threaded element with a delivery pitch equal to one diameter is the most effective pumping element.
A melt pump can be used between the extruder outlet and the die head. The pumping efficiency of the melt pump is 25% to 35%. Using a melt pump minimizes melt temperature rise, saves extruder power, and produces a more consistent extrusion flow. The cost of a melt pump can sometimes be easily justified by increasing the extruder output.
Biopolymers degrade more rapidly than most petroleum-based polymers. If the extruder is to be left standing for more than 1 to 2 hours, it is highly recommended to lower the extruder barrel heating or turn off the heating.
If the extruder is to remain heated for an extended period of time without the extruder running, clean the extruder with a low-cost heat stabilized polymer with a melt viscosity slightly higher than that of the bioresin. If the viscosity of the cleaning material is too high, it may appear as a contaminant for a few hours after restarting production.