Abstract
The chemical synthesis of polysiloxanes from monomeric starting materials involves a series of hydrolysis, condensation and modification reactions with complex monomeric and oligomeric reaction mixtures. Real-time monitoring and precise process control of the synthesis process is of great importance to ensure reproducible intermediates and products and can readily be performed by optical spectroscopy. In chemical reactions involving rapid and simultaneous functional group transformations and complex reaction mixtures, however, the spectroscopic signals are often ambiguous due to overlapping bands, shifting peaks and changing baselines. The univariate analysis of individual absorbance signals is hence often only of limited use. In contrast, batch modelling based on the multivariate analysis of the time course of principal components (PCs) derived from the reaction spectra provides a more efficient tool for real-time monitoring. In batch modelling, not only single absorbance bands are used but information over a broad range of wavelengths is extracted from the evolving spectral fingerprints and used for analysis. Thereby, process control can be based on numerous chemical and morphological changes taking place during synthesis. “Bad” (or abnormal) batches can quickly be distinguished from “normal” ones by comparing the respective reaction trajectories in real time. In this work, FTIR spectroscopy was combined with multivariate data analysis for the in-line process characterization and batch modelling of polysiloxane formation. The synthesis was conducted under different starting conditions using various reactant concentrations. The complex spectral information was evaluated using chemometrics (principal component analysis, PCA). Specific spectral features at different stages of the reaction were assigned to the corresponding reaction steps. Reaction trajectories were derived based on batch modelling using a wide range of wavelengths. Subsequently, complexity was reduced again to the most relevant absorbance signals in order to derive a concept for a low-cost process spectroscopic set-up which could be used for real-time process monitoring and reaction control.
Highlights
The stable polymer backbone of alternating silicon and oxygen atoms and the absence of carbon–carbon bonds is the basis of various beneficial properties of polysiloxanes, like UV-resistance, thermal stability and longevity [1,2,3,4,5]
The synthesis of polysiloxanes is dominated by hydrolysis and condensation reactions of monomeric and oligomeric educts, intermediates and products
Synthesis of polysiloxane was performed at three different reactant concentrations and, at three different levels of acid catalyst in order to evaluate the potential of Attenuated total reflection probes (ATR)
Summary
The stable polymer backbone of alternating silicon and oxygen atoms and the absence of carbon–carbon bonds is the basis of various beneficial properties of polysiloxanes, like UV-resistance, thermal stability and longevity [1,2,3,4,5]. Cost-reduction can be achieved by designing a specialized spectrometer “around the required wavelengths” by omitting all unnecessary components Such low-cost spectrometers proved already to be potentially suitable for analysis of chemical compositions by means of different measuring techniques [22,23]. It is evaluated to what extent a low-cost in-line spectrometer with highly specific components for the problem at hand could be suitable for real-time reaction tracking and process control by further reducing the spectral information to a few characteristic wavelengths. To mimic such a low-cost spectrometer, the batch model is recalculated with a reduced data set and its validity is tested
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