Monitoring Polymer Production Process with Fiber Optic Probes

Polymers are known to have excellent chemical resistance, strength, and elasticity. However, these very properties can hinder their chemical analysis. Therefore, the quality control techniques used for measuring additive levels in polymers vary dependent on the physical and chemical properties of the polymers.

In the polymerization process, even slight differences in the process parameters can cause large variations in the characteristics of polymer products. Hence, in order to ensure product quality and reliability, the polymer properties must be continuously determined, and the processing parameters must be adjusted through rapid feedback.

NIR spectroscopy offers structural and kinetic data in real time and so can be used for such applications. This method is fast and needs little to no sample preparation, with minimal technical expertise required. Most of these advantages can be attributed to a fiber optic probe positioned within the reactor system. This removes the requirement for sampling, and so reduces experimental error and time related to the sampling process.

Advantages of Fiber Optic Probes

NIR spectrometer systems integrated with fiber optic probes bring a new dimension to process monitoring in polymer manufacturing plants. When an NIR probe is joined to a spectrometer through an optical fiber, direct in-line and on-line monitoring can be realized without actually affecting the production process. Fiber optic probes can be utilized in adverse operating environments, whilst the analysis computer and the spectrometer can be kept safely in a process monitoring room.

Remote monitoring is also possible at large distances without causing any major impact to signal-to-noise ratios. For contact and non-contact measurements, many NIR optical probes are available, ranging from immersion probes and transmission pair probes to reflectance probes. With this range of options, NIR spectroscopy can be used on nearly all types of polymers, including solid powders, solutions, melts, and emulsions.

Finding the Right Match

In order to ensure effective process implementation for on-line/in-line process monitoring, it is important to choose the right sample interface or probe for use in the NIR system. For optimum system performance, it should be ensured that the probe optically corresponds with the optical fiber, which transmits the spectral data, as well as with the spectrophotometer.

However, when NIR sampling interfaces are used for monitoring the polymerization reaction, a number of problems are encountered, which include rapid temperature change, sustained high temperatures, high pressure, fouling of the probes, and polymer flow issues. For successful process implementation, the probe must be chosen depending on the measurement method, application, and environment.

Probe materials should not react with the materials in the process, and they must be able to endure the expected high pressure and temperatures. One way to reduce the risks induced by rapid temperature changes is to consider the relative thermal expansion coefficients of the probe materials and the window material during the development of the probe.

Probes that are commercially available in the market can be used in most polymer process monitoring applications. These are mainly made of hastelloy or 316L stainless steel combined with a sapphire window, and can withstand 5,000 psi pressure and up to 300°C temperature.

A micro transmission probe pair, or micro interactance immersion probes, are often used for measuring clear and scattering liquids. Such probes can even be used on slurries that have 15 to 20% solid content. Measurement of certain solids requires the use of specialized probes, which come with variations like an optimized 45° micro reflectance probe, or a micro interactance reflectance probe featuring purge capabilities on its collection tip. Non-contact probes can be used for other applications, such as when samples are moving on a conveyor belt.

Types of NIR Probes

Micro Interactance Reflectance Probe

Micro interactance reflectance probe is placed such that it directly contacts with the sample of interest. However, this device cannot measure the specular reflectance of a sample, but can determine its diffuse reflectance. It is suitable for solid samples like granules, powders, or slurries comprising 15% to 20% solids, and can be directly deployed to the process line through a welded flange or compression fitting. The probe is typically employed for tracking hot melt extrusion and bulk polymerization processes.

Micro Interactance Immersion Probe

The micro interactance immersion probe is mainly used for studying liquid samples in transflectance mode. It comprises a body and an adjustable high-energy mirror tip, and the sample passes through the gap between the two. The position of the mirror tip can be altered to define the pathlength for analysis. The probe can be used for studying clear to scattering slurries and liquids with below 15% solids, and can also be used in extrusion processes that are controlled by pressure and temperature and in solution phase processes. It can be directly deployed to the process reactor, or can be installed into a side-stream loop through a welded flange or compression fitting.

Micro Transmission Probe Pair with Lenses and Spacers

This probe is often used for studying liquid samples in true transmission mode. It contains two transmission probes that are positioned 180° opposite each other, with a spacer linking the probes. Using the threaded spacers, the pathlength between the transmission probes is reproducibly set. This approach can be applied to clear to scattering slurries and liquids with less than 15% solids, as in pressure- and temperature-controlled extrusion processes and solution phase processes. The micro transmission probe pair can also be deployed to the process reactor, or to a side-stream loop through a welded flange or compression fitting such as Swagelok™.

Micro Interactance Reflectance Probe with Purge on Collection Tip

The micro interactance reflectance probe is a specialized probe used for studying granules, powders, and other solid samples in environments, where there is variability in the sample amount. The probe features a spoon or a collection tip at the end to collect the sample at a specified time.

This time delay ensures that an adequate amount of sample is obtained to create a reproducible spectrum. Once the spectrum has been acquired, the sample is blown off the collection tip once the purge has been activated. This allows a fresh sample batch to be obtained and investigated. The micro interactance reflectance probe is typically used for tracking the drying process of solid samples. It can be easily deployed to a fluid bed dryer or a process line/reactor. Using a welded flange or compression fitting, the probe can be easily connected to the process line.

Optimized 45° Micro Reflectance Probe

This probe has been specially developed to ensure improved sample contact for powders and granule samples. The tip of the probe is kept at a 45° angle, which facilitates the sample to pass along the probe in a smooth and reliable manner and results in reproducible spectra. The probe can be deployed to the process line to make direct contact with the sample through a welded flange or compression fitting.

Non-Contact Probe

The non-contact probe is suitable for measuring samples passing on a conveyor belt, but can also be used for other noncontact measurement applications. This type of probe is generally used for checking curing procedures. In order to ensure optimum results, it can be deployed in such a way that the sample moves at a distance of 4 to 10” from the probe window. Table 1 shows the summary of different probes and their application.

Table 1. Summary of probe types and their application

Probe Maintenance

A common issue in polymer systems is probe fouling, which often displays as a gradual increase in the baseline absorbance over a course of time. However, proper care should be taken to ensure that the baseline increase does not lead to vagueness in the process itself, or does not cause any optical degradation of the probes.

Based on the application process, probe fouling can be dealt in a number of ways. If the baseline shifts are small, they can be prevented through the spectra’s baseline correction, but probe fouling also promotes changes in the baseline curvature and tilt.

These changes can be dealt with in the calibration modeling when there is a low level of fouling. The rate of fouling can be reduced through changes in the probe location, materials of construction, or orientation with respect to the flow direction. It is also possible to clean and remove the probes at a set frequency.

Fiber Optic Interfacing

Fiber optic cables are often used to transfer the NIR light from the instrument to the process probe. When the light scattering characteristics of the process sample increases, the number of fibers integrated in the fiber optic bundle should also be increased. This helps maintain the analytical performance of the instrument.

Clear liquids are studied using single fiber process NIR analyzers, while slightly scattering suspensions, liquid media, and drying processes are tracked with micro-bundle process NIR analyzers. For complex applications, like analysis of low-level constituents or tracking the drying process of hydrated media, full-bundle process NIR analyzers are employed.

The extent of the fiber optic interface can range from 1 to 150 m. Longer fiber optic lengths make it possible to locate a process analyzer beyond safety classified or electrically classified areas, and protect it from adverse operating conditions such as large temperature fluctuations. Table 2 shows the comparison of fiber bundle size, fiber optic interface, and measurement mode.

Table 2. Comparison of fiber optic interface, fiber bundle size and measurement mode

A multiplexed process NIR analyzer can be used for checking nine sampling points or process streams. Multiplexing not only reduces the cost per measurement point, but also reduces the total implementation expenses for a process NIR analyzer. Conversely, a risk evaluation must be made to make sure that the economic advantages are able to sustain the increased liability for each measurement point.

Conclusion

In-line high-pressure and high-temperature probes, fiber optics, and analytical instruments facilitate the use of spectroscopic methods for tracking the processes of polymer production in industrial set ups. NIR spectroscopy is a key analytical tool that can be used for optimizing process control as well as product quality, and could help deliver better and more cost-effective products to the market.

The NIR probes are highly flexible and versatile, and prevent the costs and difficulties of chemicals and reagents required for analysis. At the same time, they speed up the feedback loop by rendering real-time information for a range of polymer production applications.

This information has been sourced, reviewed and adapted from materials provided by Metrohm AG.

For more information on this source, please visit Metrohm AG.