Biopolymer compounding on a twin-screw extruder: four things you need to know | Plastic Technology

2021-11-13 02:16:47 By : Ms. Jane Chen

By 2024, driven by original equipment manufacturers and brand owners, global bioplastics production capacity is expected to increase to 4.8 billion pounds, steadily converting applications to these "green" materials. If you want to provide compounds to meet this need, here are some key processing considerations you need to know.

Biopolymers such as PHA are expected to drive the growing demand for "green materials", and some sources predict that production capacity will more than triple in the next five years. PHA is a bio-based polyester, which is biodegradable and provides good physical and mechanical properties.

Australia's Plantic Technologies Ltd. (now owned by Kuraray) is considered one of the pioneers in this industry and has worked with us to develop proprietary formulations. Today, their patented biodegradable food packaging materials are commercially successful and are used worldwide and are produced on twin-screw compound machinery. Danimer Scientific LLC, based in Bainbridge, Georgia, is another leader in this field. Danimer produces biopolymer compounds used in the production of various environmentally friendly products. Its Nodax PHA series is experiencing significant growth; for example, Bacardi is using it to replace the 80 million PET bottles (equivalent to 6 million pounds of material) the company currently produces.

Many new players with new materials and many new formulations have appeared in the field of biopolymers. The co-rotating twin-screw extruder provides excellent material mixing capabilities for all polymers (including biopolymers). Regardless of the formulation, you should treat your machinery supplier as a development partner to achieve your goals in terms of melt and mixing quality, productivity, and other parameters. Ensure that your machinery supplier has the facilities and expertise to help you test material formulations and improve processes.

As with any compound project, a certain amount of trial and error is inevitable during the process development of a new biopolymer formulation. Some were successful from the beginning; others needed more work to be commercially viable.

PHA has been commercialized for a range of applications, including Phade brand straws made from Danimer's biological compounds.

We roughly divide biopolymer compounds into three categories:

• Bio-resin compounds containing bio-resin, such as PHA, PLA or PHB;

• Biopolymer composite mixture, defined as a compound containing two or more bio-resins, starch-based polymers and/or petroleum-based polymers)

The following are some key processing considerations for the bio-resin compound and bio-polymer compound mixture on the co-rotating twin-screw extruder:

 1. Purchase a flexible extruder: design the equipment for maximum flexibility; purchase a long extruder with 48:1 or 52:1 L/D, with multiple vents and feed location options. This is a fast-growing industry, with new ideas, new or improved raw materials, new or improved additives, etc. It is much easier to open or close available vent locations, or to add or remove side feeders from available locations, rather than buying a new barrel or extending the extruder after the initial capital purchase.

 2. Remember that biopolymers are generally hygroscopic: plan to have at least one atmospheric vent—sometimes two—and at least one vacuum vent—sometimes two. Biopolymers are generally hygroscopic and require careful handling before processing. It is effective to place an atmospheric vent early in the extrusion process to minimize the hydrolysis of the biopolymer and avoid pre-drying of the material. Vacuum vent is required. Don't treat the vacuum system as a non-essential accessory; it is an integral part of the extrusion process and is usually the difference between making a good product or not. The well-designed vacuum system also solves the problem that biopolymer volatiles are usually corrosive, and is worth the price by reducing maintenance and downtime and increasing throughput.

 3. Consider that biopolymers are sensitive to shear and pressure: when exposed to overheating and/or shear, biopolymers will degrade rapidly. The shear exposure of the material is directly proportional to the extruder screw speed and inversely proportional to the gap between the extruder screw and barrel. Run the extruder at the lowest possible screw speed at a given throughput rate until the extruder torque is too high (>90%) or the volume limit is reached, as defined by the compound support inlet or exhaust.

It is possible to strategically place vents to allow air and moisture to escape, and to avoid volume restrictions by placing restrictive kneading blocks or reverse screw elements as far as possible downstream of the feed or vents. By designing the extruder screw to have a long "mild" mixing zone instead of a short intense mixing zone, high torque can be avoided. Stabilize the feeder to minimize torque fluctuations, allowing the operator to run the extruder with a higher average torque.

The biodegradable biscuit tray here is extruded and thermoformed from Plantic's PHA.

Although the co-rotating twin-screw extruder is an effective and efficient mixing equipment, its efficiency as a pump is only 8% to 15%, which means that due to the high outlet pressure of the extruder, the melt temperature rises significantly . The high outlet pressure can also cause exhaust gas flow at the exhaust port near the extruder outlet. Increasing the die hole diameter, increasing the screen size of the filter screen, and increasing the water and mold temperature in the case of underwater pelletizing, all of which can minimize the pressure at the outlet of the extruder. A conveying screw element with a pitch equal to one diameter is the most effective pump element.

A melt pump can be used between the extruder outlet and the die. The pumping efficiency of the melt pump is 25% to 35%. The use of a melt pump can minimize the melt temperature rise, save the power of the extruder and produce a more stable extrudate flow. The cost of the melt pump can sometimes be easily justified by increasing the output of the extruder.

 4. Biopolymers degrade rapidly, so purification is important: Biopolymers degrade faster than most petroleum-based polymers. If the extruder will stand for more than 1 to 2 hours, it is strongly recommended to reduce the barrel heating or turn off the heating. If you keep heating for a long time when the extruder is not running, use a low-cost, heat-stable polymer with a slightly higher melt viscosity than bio-resin to clean the extruder. If the viscosity of the cleaning material is too high, it may show up as contamination within a few hours after restarting production.  

About the author: Dean Elliott has more than 20 years of experience in plastic extrusion. He received a bachelor's degree in mechanical engineering from Durban Natal University in his hometown of South Africa. He rejoined ENTEK in 2017 after working for an interface consultant/REV material company in Prescott, Wisconsin for three years, where he led equipment installation projects and managed multiple strategic client accounts. Prior to this, Elliott worked as the extrusion laboratory manager of ENTEK for seven years, where he led all customer extrusion laboratory trials and was dedicated to helping customers solve numerous processing challenges. He is the holder of two patents. Contact: 541-259-1068;;

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