|The moisture in the granules of most engineering polymers is required to be below a certain level for processing. Sensitivity of the raw material to water, the moisture content of the material when delivered, the type of packaging and storage period are also important criteria determining moisture in the material. The moisture content in PET and PBT is critical, hence the material must always be dried to ensure that there has been no influence on the impact strength of the moulded parts. Before processing, the material should be dried to achieve optimum molded-part properties. Also, PET and PBT also pick up moisture very fast after drying in adverse climatic conditions - PET can absorb enough moisture in 10 minutes to exceed the maximum permitted moisture content for moulding of 0.02%. Fully saturated polyamide may need more than 12 hours to dry.
Infrared energy can provide an efficient method of drying moisture sensitive polymer such as PET and engineering plastics. IR drying systems for PET have had problems of dust collecting on the heaters and drum walls, but not with Novatec’s new technology -an IR crystallizer/dryer for PET regrind. The Infrared Rotary Drum (IRD) unit is a slow turning cylinder with a wide, shallow spiral channel on its interior, as well as paddles that stir the PET regrind as the drum turns. The helical channel gradually conveys the material along the axis of the drum. A stationary bank of IR emitters hangs in the center of the drum and a stream of ambient air flows through the drum to carry off moisture that escapes the hot material. Polymer passes through the drum in 8-14 minutes. In that short period of time, PET is not only fully crystallized but is also dried to a substantial degree.
Some additional drying is typically required–especially if the regrind is to be used in bottle or preform molding. Novatec pairs the IRD with a conventional twin-bed desiccant dryer to finish the job and to serve as a buffer hopper to keep the material dry until it is processed. Time in the buffer hopper also yields more uniform I.V. IRD system is faster because IR energy can penetrate up to 5mm into the PET particles. Dust collection on the heaters and drum walls has not been a problem with the IRD, which turns at only 1-1.5 rpm.
Cost of the IRD dryer system is not very prohibitive as it is comparable to that of conventional crystallizer/dryer systems for PET. Sheet producers and others that use over 10% regrinds require a separate crystallization step. Bottle makers using less than 10% regrind can crystallize that amount in a drying hopper without using a separate crystallizer. For such users, nearly doubling the capital cost to purchase an IRD/buffer system would be a questionable choice, since the payback period would be as long as two years.