On-Line Monitoring of Polymer Processing Operations Using FT-NIR Spectroscopy

Modern product management philosophies like “Total Quality Management“ or “Zero Defect Operation” share the same aim. This is to produce the highest quality, as quickly as possible and for the least possible expense. consequently, on-line monitoring of chemical reactions and production flow is now gaining increasing attention. In particular, the plastic and polymer industry requires fast, consistent, non-invasive and economic analytical methods for process control.

Features of FT-NIR Spectroscopy

FT-NIR spectroscopy possesses a unique advantage when compared to other technologies in that they offer a real-time assessment of the process on a molecular basis. A FT-NIR process spectrometer Matrix-F is shown in Figure 1.

FT-NIR Spectrometer

Figure 1. FT-NIR Spectrometer

The features of FT-NIR spectroscopy are listed below:

  • The recorded spectra relate directly to the composition of the material. With the use of fiber-coupled probe heads, the “eye” is brought directly into the area of interest without any interference in the production process. The spectrometer itself can be installed either directly alongside the measurement line or separately, in a dedicated analysis area.

  • Modern control software transmits the data over to the process control system. Corrective measurements can be performed within a few seconds in case of a drift from the working specification is detected. This is ideal since typical off-line analysis delays the actual study of the sample up to several hours, during which the production of material of unknown quality continues with associated wastes and costs.

  • The recent development of new high temperature and high pressure probes opens opportunities in a wide range of processes. The classical transmission probes, made of stainless steel or Hastalloy with sapphire windows, are suited for most translucent liquid systems. Diffuse reflection probes are preferred when dealing with opaque or scattering substances such as polymer melts, powders or pellets.

  • The direct access to the molecular structure of the material makes the calculation of parameters such as OH-number, NCO-content, acid-value or content of free monomers readily available. The analysis of physical properties is also possible. Standard applications include the simultaneous monitoring of the density and the Melt Flow Index of Polyethylene. These parameters are crucial for processes such as extruding or moulding and to be able to monitor them is therefore imperative to guarantee the quality of the processed polymers.

Monitoring of Polymer Process

The production of a polyethylene film constitutes a good example of a complete monitoring of polymer process. The progression of the polymerization can be followed with a diffuse reflection probe placed directly in the vessel.

Real-time traces of density (red) and MFI (blue) are shown in Figure 2.

Real-time Monitoring of Density (red) and MFI (blue)

Figure 2. Real-time Monitoring of Density (red) and MFI (blue)

Polymer Applications

Some common polymer applications are listed below:

  • OH-Number
  • Acid-value
  • Amine-value
  • NCO Content
  • Dry Extract
  • Residual Moisture
  • Free Monomer Content
  • Melt Flow Index
  • Density
  • Viscosity
  • Melting Point
  • Degree of Polymerization
  • Cross Linking
  • Tacticity
  • End Group Determination

Monitoring a Complete Process with FT-NIR

Material quality can be analyzed in the pipe before entering the extruder. At the extruder outlet, a transmission probe monitors the polymer before processing. Another probe provides quality control of the finished film before coiling up is performed to effectively detect irregularities and defects. Bruker Optics process spectrometers are specifically designed for harsh industrial environments.

Monitoring a Complete Process with FT-NIR

Figure 3. Monitoring a Complete Process with FT-NIR

The MATRIX-F includes advanced multiplexing options, enabling the monitoring of up to 48 positions with one spectrometer. The MATRIX-F and its peripherals are separately enclosed modules that can be built into 19” racks. A wide range of probe adaptations are available for analysing liquids and emulsions as well as powders and pellets. Dedicated process control software that handles different protocols such as Modbus, Profibus, Industrial Ethernet or OPC is available for the constant monitoring and control of a variety of processes.

Another significant application for FT-NIR spectroscopy is the quality control of the incoming raw materials. To produce high quality products it is essential that the delivered goods such as additives, stabilizers, anti-oxidants or plasticisers meet the desired quality criteria. With the Multi Purpose Analyser (MPA), Bruker Optics also offers lab-based solutions for the QC laboratory or the warehouse.

This information has been sourced, reviewed and adapted from materials provided by Bruker Optics.

For more information on this source, please visit Bruker Optics.

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