The supply of polylactic acid, a bioplastic based on plant starch, is set to explode in the next years, and demand seems sure to keep pace. Here are some tips on how best to handle the material.
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