PET Oxygen Measurement

For Preventing Discoloration

Inline oxygen measurement helps PET manufacturers maintain oxygen-free process conditions and prevent discoloration caused by unwanted oxidation. Continuous real-time monitoring supports improved process control, consistent product quality, and efficient production of high-performance PET materials.

Background

Polyethylene terephthalate (PET) is one of the most abundantly used polymers making up 18 % of the world's polymer production. Its versatile nature allows for an extensive range of bulk applications as well as niche products. Common uses include fibers, beverage bottles and building materials.

The key to its versatility comes from its repeating molecular makeup: its long chains of ethylene terephthalate. This accounts for its chemical inertness and ability to be crystallized or made into an amorphous glass - like material, depending on the chosen polymerization process. Further modifications can be made through strict control of the polymer chain length, degree of cross - linking between the chains, addition of modifying agents, and heat treatments. Thus strict reaction and process control is required to produce the specific characterizations for the de sired application.

Process

The two chief routes for PET production are through transesterification and direct esterification. In transesterification, dimethyl terephthalate (DMT) and a catalyst are heated to a reaction temperature of 125 – 220 °C in a closed vessel. Since water is a by - product and the reaction is reversible, constant removal of the water is required and can be accomplished by vacuum, inert gas or inert solvent. For direct esterification, raw material for PET is chiefly terephthalic acid synthesized from crude oil - based paraxylene, along with ethylene glycol. The reaction is performed under a pressure of 2.7 – 5.5 bar at 220 – 260°C without the need for a catalyst. This reaction also requires continuous removal of water due to the equilibrium shift. After the reaction is completed to the desired molecular weight, the remaining solvents and precursors are removed in a drying step. The resulting polymer can be spun into fiber or pelleted into small solids.

In both methods, and throughout production steps, the materials must be under oxygen - free, inert conditions. Oxidations, which can occur throughout production, will interrupt the polymerization as well as produce undesired color changes. Yellowing, which can occur at higher temperatures, is a serious problem as it will render the product unacceptable for white or clear applications. Further, in spinning, pelleting and hopper storage, yellowing can still occur, so it is important to keep all headspaces oxygen-free.

Analysis of oxygen levels in production and storage vessels are therefore required, and this is best achieved with inine measurement systems giving continuous measurements. These provide real-time values of oxygen in the vessels and do not require sampling systems. The inline sensors must be able to provide precise and accurate readings at very low levels of oxygen.

DO Sensor InPro6900

The InPro6900 is an amperometric - type trace oxygen probe which has the ability to detect down to 5 ppm oxygen in the vessels and process pipes of PET production. With a real - time response even small increases in oxygen can be detected quickly, allowing rapid remedial action. A METTLER TOLEDO M400 2-wire transmitter completes the measurement loop for integration into any control environment.

Diagnostics of sensor health are available from the InPro 6900's Intelligent Sensor Management (ISM®) technology. ISM probes feature a Dynamic Lifetime Indicator (DLI) to show the sensor's remaining lifetime. The DLI changes according to the measurement conditions and displays on the connected transmitter a value in days before sensor replacement will be required. Further, the Time To Maintenance tool and Active Calibration Timer show how many days until servicing will be needed.

InTrac 781 Retractable Housing

For ease of cleaning, an InTrac 781 retractable housing is used. It includes a flushing chamber for rinsing any potential fouling buildup on the sensor. This ensures consistent performance without process interruption.

Oxygen contamination can have a significant impact on PET quality, affecting polymerization efficiency and causing unwanted discoloration. Download the full Application Note to learn how continuous inline oxygen measurement helps detect oxygen ingress at trace levels and supports stable process conditions throughout production and storage.

Explore how real-time oxygen monitoring can help prevent yellowing, improve product consistency, optimize operational control, and protect the quality of PET materials for demanding applications.