Trace Moisture Measurement in Polyethylene Terephthalate (PET) Bottle Manufacture

APPLICATION NOTE

Polyethylene Terephthalate (PET) is a widely used thermoplastic polymer, especially in the production of bottles for beverages, food, and other consumer goods. Ensuring optimal product quality in PET bottle manufacturing requires precise control of several parameters, with trace moisture measurement being one of the most critical.

What is PET and How is it Used?

PET is a strong, lightweight, and transparent plastic. Its key properties include:

High strength-to-weight ratio
Excellent chemical resistance
Good barrier properties against gases and moisture

Applications of PET

Beverage bottles (water, soft drinks, juices)
Food packaging
Personal care containers
Synthetic fibres (polyester)

Moisture Problems in PET Bottle Manufacture

Why Moisture Matters

During PET processing, moisture content must be strictly controlled. PET is hygroscopic, meaning it absorbs moisture from the environment. If PET pellets contain excess moisture during melting and molding, several issues can arise:

Hydrolytic Degradation: Water reacts with PET at high temperatures, breaking polymer chains and reducing molecular weight.
Loss of Mechanical Strength: Degraded PET leads to brittle bottles with poor impact resistance.
Surface Defects: Increased haze, bubbles, and discoloration in finished bottles.
Reduced Barrier Properties: Compromised ability to protect contents from oxygen and CO₂ ingress.

Typical Moisture-Related Defects

Defect Type	Cause	Impact
Brittleness	Chain scission by water	Cracking, breakage
Haze/Discoloration	Degradation by hydrolysis	Poor appearance, off-color
Bubbles	Vaporization of moisture	Weak spots, leaks

Solutions: Trace Moisture Measurement and Control

Pre-Drying PET Pellets

PET resin is typically dried to below 50 ppm (0.005%) moisture before processing, using desiccant dryers.

Moisture Measurement Techniques

Capacitance Sensors: Fast, inline monitoring.
Dew Point Meters: Monitor dryer performance.

Process Control

Continuous monitoring ensures consistent low moisture.
Automated alarms and feedback loops prevent processing of wet resin.

References to Applicable Standards

ISO 15512: Plastics — Determination of Water Content

ASTM D7191: Standard Practice for Quality Assurance of PET Resin

Trace moisture measurement is essential in PET bottle manufacturing to prevent product defects, maintain mechanical strength, and ensure compliance with industry standards. By implementing robust moisture control and monitoring systems, manufacturers can achieve consistent product quality and reduce waste, supporting both operational efficiency and customer satisfaction.