(255cm) Assessment of Wavelength Exponent Method for Monitoring Inverse Miniemulsion Polymerization of Acrylamide Using NIR Spectroscopy

Acrylamide is a solid, water-soluble monomer precursor of polyacrylamide, with different applications: flocculants, adhesives, viscosity control agent for oil, chemicals for the paper industry, mining processes. Polyacrylamide can be produced using differences polymerization techniques, such as solution polymerization, inverse emulsion, and inverse polymerization in microsuspensionor micro emulsion. However few studies are reported in the literature related to inverse miniemulsion polymerization.

The compartmentalization of the radicals in submicron-sized particles provides several advantages for emulsion and miniemulsion polymerization techniques compared to bulk, solution and suspension polymerizations, such as higher reaction rate and higher molecular weights. In addition, the resulting product is a dispersion of submicrometric polymer particles and there are fewer problems due to heat transfer or mixing during the polymerization since the viscosity of the continuous phase is low.

It has been seen that, inverse miniemulsion polymerization of acrylamide shows a high reaction rate, due to high ratio between propagation and termination rate of acrylamide, which become the monitoring of proprieties such as, droplet/particle diameters, and conversion difficult. In addition, these properties are important to know the course of reaction, because it can be an indicative of loss stability of reaction. Thus, monitoring polymer properties during the reaction is important to eliminate or reduce variations in product quality or other failure during the process.

There is also a need for improving existing technology to reduce costs and increase process safety, maintaining or improving product quality, and reducing the amount of residual volatile organic compounds generated in conventional analytical methods. These conventional techniques usually are time-consuming and not suitable for in-line and real-time monitoring. Therefore, spectroscopic techniques, combined with optical fiber sensors, are in an outstanding position to monitor heterogeneous processes.

Among them, Near Infrared (NIR) Spectroscopy, which comprises the spectral region from 13000 cm-1 to 4000 cm-1, is a widely used technique to monitor polymerization processes. NIR spectroscopy techniques have the advantages of ease of handling and rapid measurements, the ability to monitor simultaneously different polymer properties, as well as the possibility of using a multiplexer system. However, it is an indirect method, so the fitting of multivariate calibration models is required to relate NIR spectra with properties of interest, such as monomer concentration, conversion, and droplet/particle diameters throughout the reactions. This calibration step is time-consuming and the fitted model is valid only for being used in the same conditions in which the model was built.

A method called Wavelength Exponent Method is shown in literature as a simple and fast technique for evaluate changes over time in properties of heterogeneous systems. The extinction of light by emulsions or dispersions is the result of light absorption by the continuous and dispersed phases plus scattering. For a nonabsorbing system, the measured spectral absorbance can be directly related to scattering by the suspended particles. The extinction efficiency of this scattering depends on the particle size parameter and the refractive index of both phases, evaluated at the correspondent wavelength. For dilute dispersions consisting of monodisperse spherical nonabsorbing particles significantly smaller than the wavelength of the incident light, scattering is described by the Rayleigh scattering regime.

Based on this theory, the so called Wavelength Exponent can be deduced and determined from spectroscopic measurements and its calculated value can be related to particle size. Therefore, his technique can be applied to evaluate changes in heterogeneous systems by observing changes in the exponent value. Usually this technique is applied to the visible range of the spectrum, but in this work it was observed that it can also be successfully used in the NIR range.

For using this method in the monitoring of a polymerization reaction, it is necessary to choose a wavelength range where there is no absorption of any components of the system, so the measured absorbance will be due to scattering only. This means that only physical information of the system is used, so changes in the chemical composition should not affect the results. This is an interesting advantage in the method, besides its simplicity, for the evaluation of particle size during the polymerization reaction. If no calibration step is made, the wavelength exponent method still can be used as a qualitative evaluation of changes in the properties of the reaction over time.

In the studied system, acrylamide polymerization by inverse miniemulsions, there are two types of particles present: monomer droplets and polymer particles. Therefore, if a droplet size analysis were made during the reaction, the measurements will not indicate the size of the monomer droplets or polymer particles separately, but a combination of both, making it difficult to monitor the reaction over time.

Although only a qualitative evaluation where made, the monitoring of the wavelength exponent over time, calculated from the measured NIR spectra during the acrylamide polymerization reaction, has shown interesting results. Besides the particle size dependency of the exponent, it was observed that, when the reaction conversion rate suddenly increases, there also is a sudden change in the behavior of the wavelength exponent. The founded results are supported by the behavior found in the droplet size measurements, indicating that this could be an interesting technique for monitoring such systems.