Polymers are organic materials that are characterized by unique properties, such as tensile strength and viscoelastic behavior under deformation. As a result, these materials are used extensively in many different fields. Stringent demands on the quality of polymer products, paired with the pressure to reduce the cost of processing and production, have resulted in the need for fast, accurate and reliable monitoring. In analytical methods, extensive sample preparation is required, which takes considerable amount of time. These techniques cannot be effectively implemented in a quality control or process environment, as a result precious time and materials are wasted. In order to ensure efficient process control a measurement method that is fast, requires little or no sample preparation and with minimal technical expertise should be used. The technique should also be robust, consistent and easily automated.
Near-infrared (NIR) spectroscopy can demonstrate a fast and accurate measurement technique. NIR analyzers are capable of providing instant results, often in less than 60 seconds. The use of a probe can either totally eliminate, or keep sample preparation to a minimum. A high signal-to-noise performance, with the scan speed further improve the NIR analyzers. Used for monitoring multiple components simultaneously, or placed on-line or in-line will further improve the speed of the analyzer. Routine sampling of the process, which can involve safety risks and be time-consuming, can also be eliminated by in-line analysis. Providing highly accurate results, the NIR analyzers have high stability and eliminate variability that could arise from sampling and sample preparation.
NIR for Polymer Polymerization Processes
It is essential to monitor polymerization reactions from the research phase through to the production. At the very basic level it provides information on kinetics and mechanisms necessary for new material development, at the pilot plant level it can be used in optimizing reaction conditions. At the industrial reactor level, when NIR is used in a feedback-control-loop, better product quality can be obtained along with considerable savings of reactor, energy, non-renewable resources, and personnel time. To implement an effective NIR strategy, the target properties, the type of polymerization process, and measurement conditions should be considered.
Polymerization Processes
NIR spectroscopy can be utilized for bulk, suspension, emulsion and solution polymerization monitoring. It provides data on composition and distribution of copolymers, conversion of monomers, average particle sizes, molecular weight averages, etc.
Bulk Polymerization
In bulk polymerization, the reaction is carried out in the absence of diluent, solvent, or other similar materials. Bulk polymerization can adapt to copolymerization with other compatible comonomers, and as a result leads to high product purity. This process can be utilized when the polymer does not form wall deposits, or cross-linked gels, which would contaminate the continuous polymerization apparatus. Heat should be removed to prevent the formation of explosive compounds.
Solution Polymerization
Solution polymerization is performed in the presence of an inert solvent and initiator, unlike bulk polymerization. Benefits in solution processes come from the low viscosity and homogeneous properties of the reaction, while the issues with this process are a lower reaction rate, productivity and molecular weight average of the final product.
Suspension Polymerization
The manufacturing of polymer beads is often done using suspension polymerization. In a standard suspension process, an organic phase, containing comonomers, an initiator, and the final polymers, are suspended in an aqueous phase containing residual monomer and additives. The reaction then takes place in this heterogeneous mixture. In suspension processes the polymer material is easier to purify, however it is critical to control the average particle size and particle size distributions of the end polymer particles. These variables can affect processing, compounding, and bulk-handling properties.
Emulsion Polymerization
Emulsion Polymerization is an industrial process, widely used for producing different types of synthetic polymer latexes or colloids. These products are employed in many applications such as paints, coatings, synthetic rubber, binders, and adhesives. A standard emulsion polymerization recipe includes monomers, water, water soluble initiator, additives, and surfactant. This results in a heterogeneous reaction mixture, containing submicron solid polymer particles, and dispersed within an aqueous medium. The polymer’s particle size relies on the size of the droplet and the rate of agitation. This means, continuous agitation is a required in emulsion polymerization.
Emulsion polymerization is often monitored by sampling and off-line measurement due to online probes being at risk of becoming coated or fouled. Also emulsion polymerization systems are considered complicated as they involve various phases (aqueous phase, monomer droplets and polymer particles) and compounds (aqueous, monomer, polymer, initiator, stabilizer and buffers). In addition, the spectra they yield can be difficult to interpret, and this can make NIR monitoring more challenging, in comparison to other polymerization method.
NIR Analysis Strategies
The current generation of NIR instruments can be installed into laboratories, at-line or directly into a process stream, dryer, extruder or reactor. The most appropriate NIR measurement mode and location of the NIR analyzer is decided by the method's selectivity and sensitivity for the required analytes, the optical properties of the sample, the duration of the process run and monitoring and control requirements. Fiber optic probes are used for NIR process monitoring, as they help in making direct measurements in different types of samples and environments, even in remote locations.
In-Line Analysis
In in-line analysis, fiber optics are used to interface a NIR analyzer directly to the process. The probe (constructed of stainless steel or other materials) is placed into a port installed in the process stream or vessel. This analyzer configuration can provide results in <10 seconds, and can perform a particular analysis on a specific sample type. The direct interface offers unattended, optimized, near real-time analysis on specific media, and requires minimal supporting hardware. A drawback of this approach is that it is not possible to carry-out maintenance, unless the process is fully shut down. In-line analysis is ideal for closed-loop monitoring and control strategies for scale-up and manufacturing operations. An example of a typical application for in-line analysis would be; polyester batch reaction, analyzed using an interactance immersion probe.
On-Line Analysis
In on-line analysis, a sample loop is used to interface the NIR analyzer to the process stream. This process is dedicated to performing a specific analysis on a particular sample and generates results in <10 seconds. NIR spectral measurements are carried out on a continuous sample flow as it travels via a flow-cell. Side-streams are used for sample conditioning like filtering, heating, or degassing. The side-streams enable maintenance, and calibration and test samples to be analyzed, while the process is in operation. Similar to in-line monitoring, on-line monitoring offers unattended, optimized, near real-time analysis on certain media, and is suitable for closed-loop monitoring, and control strategies for manufacturing and scale-up operations. A typical application of this analysis would be; polyurethane production process, analyzed using an interactance immersion fiber optic bundle probe
At-Line Analysis
In at-line analysis, the NIR analyzer is not interfaced directly, but instead located close to a process stream. This approach is suitable for conducting a specific analysis on a specific sample type, where on-line or in-line monitoring cannot be performed, e.g. emulsion polymerization. However, the downside of at-line analysis is that manual sampling has to be done, meaning results can be delayed by several minutes or more. The analyzer should also meet proper classifications, like IP55-EMA12. At-line analysis can be used in control strategies, process monitoring, as well as in manufacturing operations. Typical applications of at-line analysis include; polyacrylamide emulsion, measuring residual monomer content via reflectance measurement.
Instrumentation for Polymerization Reaction Monitoring
With process NIR analyzers, near real-time chemical process data can be obtained whilst operating in adverse production conditions. This means, the type of sample and process conditions govern the process sample interface. Clear to opaque solids and liquids are studied by contact reflectance and transmission probes. Non-contact reflectance measurements are carried out on materials transported in conveyor lines and hoppers.
Once an interaction has occurred between NIR light and the sample, various technologies exist for its measurement, analyzing the spectrum by frequency for qualitative or quantitative analysis. One class of instruments observes bands of frequencies, providing spectral coverage over a narrow spectral region (50-100 nm), this includes broadband, discrete filter photometer and light-emitting diode (LED) based instruments. Another band gives more full-spectrum and continuous coverage by scanning across the spectrum. Included in this band is diffraction grating, interferometer, diode-array and acousto-optic tunable filter (AOTF) based instruments.
Sensitivity and selectivity need to observe the required analytes, reliabilty, ease-of-use and implementation needs are all factors that are considered when selecting the appropriate technology. The instruments ability to operate in harsh production environments and vibration resistance should also be thought about.
Method Calibration
It should be noted that NIR spectroscopy does not directly provide quantitative analysis of chemical mixtures. In order to apply NIR spectra for quantitative analysis, a link should be established between the concentration and the measured data. Regression analysis comprising first-order or higher-order polynomials, called calibration curves, is a standard model to express this relationship. These calibration curves correlate the analyte’s concentration in response of the spectrometer.
Once the model has been built, the model quality should be evaluated by observing model parameters and validation through a separate data. Before being installed into the process steam, an online analyzer should be calibrated, Grab samples, and/or synthetic samples can be used to accomplish this off-line. It could also be achieved by installing the analyzer in a lab-scale reactor, or in a semi-works or pilot plant. However, the analyzer can be calibrated on-line if certain options are not possible.
Method Standardization
A combination of internal performance standards are used by NIR analyzers to sustain the stability and response of the instrument. Also employed are NIST-traceable external standards,directly placed at the sample location to exactly match the wavelength and photometric response for all the analyzers. When the performance for all instruments is matched accurately, a qualitative library or a quantitative calibration model built on a single NIR analyzer can be leveraged to estimate the qualitative/quantitative results on all analyzers. This approach can be used on the same analyzer following service, without manipulating any other data.
Method Maintenance
Changes in raw materials, process improvements, or other "uncontrolled" factors could potentially cause the performance of a NIR method to be compromised over time. Another possibility is malfunction of the NIR instrument. It is suggested for routine control tests to be conducted on a regular basis to track both the analyzer and the process, and thus sustain confidence in the precision of the NIR measurement (Figure 1).
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Figure 1. Control chart for monitoring the NIR analyzer and method performance to identify "out-of-control" situations.
NIR for Polymer Extrusion Processes
Polymer extrusion is the standard process used for producing a variety of plastic products, such as key vehicle components, microscale implants, etc. It is critical to control this process, which can be achieved through NIR spectroscopy. NIR spectroscopy can be used for studying the additive concentration, polymer composition, and flow properties throughout the extrusion process. An immersion, transmission or reflectance probe can be utilized based on the type of sample to be studied. Figure 2 shows the measurement of differences in talc, UV additives, ethylene-octene copolymer, and polypropylene performed through a diffuse reflectance probe.
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Figure 2. Spectral changes due to a) high and low UV Additive b) high and low EAO c) high and low talc.
NIR for Polymer Curing Processes
The final characteristics of thermosetting resins rely both on the curing process, and the chemical nature of the monomers used. Light curing resin technology enables the use of urethane resin, vinylester, and polyester in innovative applications, and the same can be developed through new techniques. This paves the way for new opportunities in both composites and coating applications. The resins can easily overcome the drawbacks seen in standard adhesives, therefore the demand for UV-curing resins is growing exponentially. Reduced environmental compliance issues and wastage are additional benefits provided by the light curing resin technology. NIR spectroscopy can be used to track the curing process in real-time. This provides new opportunities for studying curing kinetics, validating the extent of curing, and optimizing the curing conditions. Figure 3 shows how a non-contact probe is used to acquire the NIR spectra of resin coated fiber during the monitoring of the curing process.
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Figure 3. Curing process monitoring using a non-contact probe.
Conclusion
This article has explored how NIR process spectroscopy can be effectively used to acquire a complete understanding of polymeric materials, including the molecular changes that occur during processing, in a cost-effective manner. In this method, chemicals or reagents are no longer required, eliminating the costs related to procurement and disposal of consumables. Component composition monitoring and trend observation deliver a better understanding on whether a process is indeed under control. This facilitates closed-loop control of certain measurements of process variables or material properties, and consequentley enhances batch-to-batch yield and consistency. This removes batch failure and leads to a reduced waste of raw materials. As a result, NIR process monitoring serves as a suitable tool in polymer manufacturing processes.
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This information has been sourced, reviewed and adapted from materials provided by Metrohm AG.
For more information on this source, please visit Metrohm AG.