Biopolymer compounding on twin-screw extruders: four things you need to know
Many new international players have emerged in the biopolymer sector, 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 equipment supplier as a development partner to achieve your goals in terms of melt and extrusion quality, productivity and other parameters. Make sure your 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 commercialised.
Global bioplastics production 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 compounds to meet this demand, here are some key processing considerations you need to know.
Buy an extruder with greater adjustability
There are a number of extruders that have been designed to be extremely flexible. You can purchase long extruders with 48:1 or 52:1 L/D, they have multiple exhaust ports and feed position options.
Because biodegradable plastics is a fast moving industry, new ideas, new or improved feedstock materials, additives etc. are often introduced. It is much easier to open or close the available exhaust positions, or to add or remove side feeders from the available positions, than to purchase new barrels after the initial capital purchase or to expand the extruder.
Keep in mind that biopolymers are usually hygroscopic
Plan for at least one atmospheric vent (sometimes two) and at least one vacuum vent (sometimes two). Biopolymers are usually hygroscopic and need to be handled with care prior to processing. Placing an air vent early in the extrusion process is effective to minimise hydrolysis of the biopolymer and to avoid pre-drying of the material.
Vacuum venting is essential. Do not consider the 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 or not. Carefully designed vacuum systems also address the often corrosive nature of biopolymer volatiles, which can pay for themselves through reduced maintenance and downtime and increased productivity.
Biopolymers are sensitive to shear and pressure
Biopolymers degrade rapidly when exposed to excessive heat and/or shear. The shear exposure of the material is directly 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 supporting the feed or vent port.
Volume limitation can be avoided by strategically placing the 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. Stabilising the feeder to minimise torque fluctuations allows the operator to run the extruder at a higher average torque.
The co-rotating twin-screw extruder, although an effective and efficient mixing machine, is only 8 to 15 per cent efficient as a pump, which means that the high outlet pressure of the extruder can lead to a significant increase in melt temperature. 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 minimise the extruder outlet pressure. Threaded elements with a delivery pitch equal to one diameter are the most efficient pumping elements.
A melt pump can be used between the extruder outlet and the die head. The pumping efficiency of a melt pump is between 25% and 35%. The use of a melt pump minimises 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 rapidly, so decontamination is important
Biopolymers degrade more quickly than most petroleum based polymers.If the extruder is to be left to stand for more than 1 to 2 hours, it is strongly recommended to reduce the barrel heating or turn it off.
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 stabilised polymer with a melt viscosity slightly higher than that of the bio-resin. If the viscosity of the cleaning material is too high, it may appear as a contaminant within a few hours after restarting production.