Una-Dyn from Woodbridge, Virginia USA, a specialised supplier of materials drying and handling equipment have resolved the issues associated with the pre-drying of PLA (Polyactic Acid) resin prior to processing PLA extruded sheets used for thermoformed packaging and PLA injection moulded preforms as used for PLA bottles.
PLA resin is being increasingly used in packaging applications and is derived entirely from renewable resources such as corn.
Virgin PLA resin, is crystallized and dried to 400-ppm moisture level prior to leaving the production plant. The drying process of the PLA resin is the most critical stage of the raw material preparation, as 250 PPM or less water content is necessary to ensure product quality. This can only be accomplished with the tight control of temperature, with the corresponding exposure time and air quality. The equipment required to dry PLA consists of a Dehumidified Hopper Dryer with high temperature process options. There are four principle factors involved.
- Adequate airflow is necessary to carry thermal energy to the resin, and provide sufficient air exchange to carry away all of the moisture.
- The dryness or dewpoint of the air entering the hopper must be -40°F dewpoint. The reason for the low dewpoint is to enable any water content of the resin to move immediately to vapour and be carried away on the process airflow.
- The selection of a proper drying temperature is critical to ensure the resin does not fuse in the vessel, while still ensuring that it removes all water. The normal temperature is 160°F for four hours.
- The drying hopper used is the most important element. If the vessel is too small, the resin will not dry properly, alternatively if oversized, it could melt the resin. The hopper must also provide equal treatment of each pellet that enters the vessel. The best quality dry air is only useful if it is distributed to each pellet being dried, and the air diffusers must move the air equally to all parts of the vessel.
Una-Dyn also recommends the use of a Datalogger for the process dryer which enables the processor to monitor key performance data and profiles of the dryer.