DAK Americas Array 9921M - Manual (Page 2)

Array
™ 9921M
Product Information
polyethylene terephthalate resin
DAK Americas LLC
Gateway Corporate Center, Suite 210
223 Wilmington W Chester Pike• Chadds Ford, PA 19317
1-888-738-2002 • fax: 610-558-7385 • www.dakamericas.com
PDS-9921M Rev 06/11
DAK Americas
ISO 9001:2008
#10004457
Material Drying
Proper drying of polyethylene terephthalate (PET) is essential to produce a high quality part (container,
film, etc.) with optimum physical properties. PET is hygroscopic, meaning that when it is exposed to
humid atmospheres, it will absorb moisture. In PET the moisture is not only on the surface but diffuses
slowly through the whole pellet and is firmly held by molecular attraction. Before processing the PET,
this moisture must be removed. Carefully controlled drying of all PET is an essential requirement for
optimum processing performance and final product properties. If drying is not carried out properly, to
the known requirements of the type of PET in use, then deficiencies in process and product will result.
The deficiencies will be impossible to remedy by later process changes. These defects arise in the
injection molding or extrusion processes because at PET melt temperature (250°-280° C) any water
present causes hydrolytic degradation of the PET, almost instantaneously, with the resultant loss in
intrinsic viscosity (IV). Significant drops in IV cause loss of process control and reductions in end
product properties.
Drying of PET polymer involves the diffusion of absorbed moisture from the interior of the polymer chip
to its surroundings and, subsequently the removal of moisture from the bulk of polymer chips. Moisture
removal can be achieved by heating the polymer chip under dry air or vacuum. In an air drying system,
heated dehumidified air flows up through a chip bed and returns to the dehumidifier.
The key
requirements for a reliable drying process are:
Drying temperature: The ACTUAL chip temperature should achieve between 300° and 340° F
measured at the dryer exit.
Dehumidified air temperature: Correctly designed equipment should operate at temperatures up to
340° F measured on entry to the dryer hopper, with an absolute maximum of 370° F to prevent possible
discoloration.
Dehumidified air dew point: This should not be allowed to rise above –34° F and should preferably be
–40° F or lower, measure after the desiccant bed. Always check the correct regeneration temperatures
are being used.
Dehumidified air flow through the chip bed: Most dryers operate at around 1 cfm of airflow per 1lb/hr of
PET chip as a minimum requirement, with the airflow at the correct temperature and dew point.
Chip residence time (drying time): DAK recommends a chip residence time for Array ™ PET of not less
than four hours and preferably six hours. This is the theoretical drying time, which is calculated by
dividing dryer capacity in lbs. by throughput in lb./hr. Extended periods of high temperature can
adversely affect the polymer processing conditions. In the event of a stoppage for an extended period,
dry polymer can be stored in the dryer-hopper by reducing the air temperature to 240° F (or even
lower).