Why accurate moisture measurement is essential for the quality of polyethylene terephthalate (PET)

Polyester polymer chips such as polyethylene terephthalate (PET) are hygroscopic, meaning that they absorb moisture from their surrounding atmosphere. Too much moisture in the chips causes issues during the injection moulding and extrusion processes. As the plastic is heated, the water contained within it hydrolyses the PET, which decreases both its strength and aesthetic qualities.

Caption: Diagram to illustrate Easidew Online in use in a polymer chip drying application

This means that before PET can be processed in a molding machine, as much moisture as possible must be removed from the resin. In atmospheric conditions the resin can contain as much as 0.6% water by weight.

How are pellets of PET dried before processing?

The chips are loaded into a hopper, hot dry air at around 50 °C dew point is then pumped into the bottom of the hopper so that it flows up through the pellets, removing moisture on its way. The hot, wet air leaves the top of the hopper and is first run through an after-cooler, because it is easier to remove moisture from cold air than hot air. The resulting cool, wet air is then passed through a desiccant bed. Finally the cool dry air leaving the desiccant bed is re-heated in a process heater and sent back through the same processes in a closed loop. Chip moisture content must be less than 30 ppm before processing. Any water present when the PET is heated rapidly hydrolyses the polymer, thereby reducing its molecular weight and damaging its physical properties.

One of the most important physical properties of PET is its intrinsic viscosity (IV). The IV of the material is dependent on the length of its polymer chains: the longer the chains are, then the higher the IV will be, and the stiffer the material will be. The wetter the chips are when they are heated, the lower their IV will be, and consequently the physical rigidity & integrity of the polymer will be reduced, and the appearance will be affected.

How can polyethylene terephthalate be dried efficiently?

Because drying is all about removing moisture, effective moisture measurement of the process is the best way to make sure it’s working efficiently. This will save energy, reduce cost and ensure final product quality. Reducing energy consumption will often also help to reduce the overall carbon footprint of the product.

There are two techniques for measuring moisture in the drying process: online measurements, and spot checks. These are described below, with the recommended PST products.

1. Online measurement

Individual dryers are continuously monitored to ensure that the air supply to the PET is better than the specified dew-point temperature limit of 50°C dew point and so assures that the chip material is dried efficiently.

We offer two products suitable for this type of installation:

Caption: Self-Calibrating Trace Moisture Analyzer - Michell QMA401

Michell QMA401 where precision measurement with automated internal calibration is required, then the QMA401 Precision Moisture Analyzer can be installed near the inlet of the drying hopper, to check for leaks in the air path from the dryer. The dual alarm contacts can be linked to an indicator to warn maintenance staff when the dry air supply begins to deteriorate, or to act as a trigger for a PLC system.

Caption: Dew-Point Hygrometer - Michell Easidew Advanced Online

Michell Easidew Online and Michell Easidew Advanced Online provides a cost-effective alternative. It would also be mounted near the hopper inlet, and its dual alarm system can be used to provide an air quality warning.

2. Spot checking

Using a portable hygrometer to perform spot checks at regular intervals offers a cost-effective assurance of product quality. With easy operation and fast response to industrial standard dew points of less than 50°C multiple drying facilities can be monitored within reasonable time frames by maintenance personnel at low costs. Where particulate contamination (dryer desiccant or polymer dust) is present at the sampling point, in line filtration is recommended.

The MDM300 or Easidew PDP can be used to check the efficiency of a drier under operational load.

Get support and advice for your drying application

Our expert application and sales engineers will be happy to advise you with questions about moisture measurements in a wide range of dryer control applications. Contact us today.