For all of its sustainability benefits, handling semicrystalline plastic polylactic acid (PLA) is no cakewalk. For one thing, the material readily absorbs moisture from the atmosphere. It’s also temperature sensitive. To preserve properties and characteristics intended by the resin manufacturer, it’s important that processors minimize exposure of PLA to ambient (undried) air and adhere to certain other basic processing practices.
How best to handle and store PLA? Normally the material is crystallized and dried to moisture levels below 400 ppm by the supplier prior to shipping. However, if it is not kept in a sealed container, and depending on local conditions, PLA can pick up enough moisture in 5 minutes to defeat most of the benefits of drying. That’s why it’s usually delivered in moisture-resistant containers, including foil-lined boxes, which can prevent moisture regain during shipping and storage.
Still, it needs to be handled properly at all stages to minimize moisture regain. Silo storage is not recommended except in arid regions like the southwestern U.S., or unless the silo is specially designed and fitted with a dehumidifying dryer large enough to handle the ambient air that enters the silo during loading and unloading. Regardless of where or how the material is stored, temperatures should be controlled to below 122ºF (50ºC).
No matter the storage, PLA should be handled as little as possible prior to processing, and any conveying should be done with dried air. Ideally, it should be conveyed directly from the dryer to the processing equipment.
Proper drying: It’s not optional
The presence of even a very small amount of moisture during melt phase processing will hydrolyze PLA, leading to a reduction of molecular weight and a degradation of mechanical properties. This can cause sheet brittleness, internal holes, sagging of the web at the die exit, or other process and quality problems. With PLA, proper drying is not optional. It is absolutely essential. Resin manufacturers recommend that PLA be dried less than 250 ppm before processing. In fact, moisture levels below 200 ppm may be needed in order to ensure a reasonable safety margin and, in some cases, moisture levels below 50 ppm are recommended.
Stable and reliable ppm counts as low as 50 can only be achieved in a dehumidifying drying system capable of producing low-dewpoint drying air at a precise temperature, and holding the PLA in that controlled atmosphere for an extended period of time. We argue that “quick-dry” processes, such as those that claim drying times of 40 minutes or less, cannot approach this critical low-ppm level.
The good news is that most sheet applications under controlled conditions tolerate moisture levels of 200 ppm. But even at 200 ppm, you have to be extremely careful. A dryer that is more efficient than necessary may cost more to operate. However, a dryer that doesn’t meet the necessary ppm count as defined by the process will cost you your product and possibly your customer. It comes down to a careful economic balance of capital cost vs. performance.
Drying PLA is no different from drying any other hygroscopic material except that it is temperature sensitive. Depending on the specific grade, most manufacturers recommend drying crystallized PLA at 150ºF-190ºF (65ºC-87ºC) using dehumidified air with a dewpoint of -40ºF. Amorphous PLA dries at a lower temperature.
Precise control of temperature, dewpoint, drying time, and drying airflow are critical to achieving the properties and performance intended. Temperature is most important. If the drying temperature is too low, the PLA pellets will not dry as readily. If the temperature is too high, the material may soften and agglomerate in the drying hopper.
Dewpoint is the next most important variable, since it determines how dry the material eventually becomes. If the dryer cannot produce a stable, low dewpoint, it doesn’t matter how long the resin dries; it will never reach the low moisture levels required for optimum property development.
Finally, the design of the drying hopper should be such that every pellet is exposed to the drying air for the time required. This means the hopper must be large enough to provide the necessary residence time, and airflow is adequate to create the proper temperature and dewpoint conditions for drying.
Crystallizing amorphous PLA regrind
Virgin PLA, like PET, is crystallized and does not need to be crystallized again before drying and processing. However, once the crystalline pellets are melt processed, the crystals are destroyed and the material reverts to its amorphous state and must be recrystallized by heating it past its glass-transition temperature of 140ºF (60ºC). As the temperature approaches the glass-transition point, the PLA begins to soften, which is why it needs to be constantly agitated to prevent agglomeration. As the temperature continues to rise, the PLA hardens again into its crystalline state. In this crystallized condition, it can be dried at higher temperatures like any other hygroscopic polymer.
Some processors, however, choose to eliminate the crystallization step and simply dry the amorphous regrind and the crystalline virgin together at temperatures as low as 130ºF (54ºC). This process can take twice as long as conventional drying, but it eliminates the need for a crystallizer.
There are three types of commonly used crystallizers. By far the most common is a vertical agitating hopper with convection heating. In these systems, the crystallizer directs heated air to the material through the bottom spreader cone of the hopper. Constant agitation prevents agglomeration during heating. Among their advantages is their space-saving design, relatively low cost, initiation of the drying process concurrent with crystallization, and integration with most conventional drying systems. On the downside, it can take up to 40 minutes for these to reach full production levels—but once this level is reached, the process is continuous. Energy use can also be an issue, but actual consumption varies widely depending on the application.
Horizontal conduction heating units rely on high pellet-to-metal contact area to heat the material, and constant particle motion to eliminate agglomeration. Advantages of these are trouble-free operation once the process window has been identified, and their lower energy demand. But the initial equipment cost can be high.
The third type, fluid bed crystallizers, is essentially a modified pellet classifier that uses heated air to crystallize regrind. Typically used for pellets, these have also been used successfully for ground flake. They have been used extensively for crystallizing PET and PET copolymers. There is no known commercial experience with PLA to date; however, successful laboratory tests have been performed with all grades of semicrystalline PLA.
Relatively new on the scene is infrared crystallizing and drying. Material is metered into a horizontal tube, where it is conveyed lengthwise. Residence time is determined by the rotational speed of the drum, and the rotating action agitates the material while its surface is irradiated with infrared energy. Among the advantages of these are a short startup time (generally 10- to 20-minute residence), potential energy savings, and the ability to be coupled to a finishing dryer for low-ppm moisture level requirements.
These have their downside, too. Vacuum conveying can draw moisture-laden air from the chamber interior. They can be sensitive to flake size distribution (smaller flakes may melt), and results can depend on ambient air conditions and starting material moisture levels. They also aren’t cheap and take more floor space than other options.
Clearly there are plenty of options when dealing with PLA, and as more processors develop more experience with the material, expect the body of knowledge about this in-demand material to increase quickly.
About the author: Jamie Jamison is product manager, dryers, at The Conair Group, a leading supplier of auxiliary equipment for plastics processors. —Edited by Matt Defosse