Coating process optimization through in-line monitoring for coating weight gain using Raman spectroscopy and design of experiments

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In this study the authors developed a real-time Process Analytical Technology (PAT) of a coating process by applying in-line Raman spectroscopy to evaluate the coating weight gain, which is a quantitative analysis of the film coating layer. The wide area illumination (WAI) Raman probe was connected to the pan coater for real-time monitoring of changes in the weight gain of coating layers. Under the proposed in-line Raman scheme, a non-contact, non-destructive analysis was performed using WAI Raman probes with a spot size of 6mm. The in-line Raman probe maintained a focal length of 250mm, and a compressed air line was designed to protect the lens surface from spray droplets. The Design of Experiment (DOE) was applied to identify factors affecting the Raman spectra background of laser irradiation. The factors selected for DOE were the strength of compressed air connected to the probe, and the shielding of light by the transparent door connecting the probe to the pan coater.

To develop a quantitative model, partial least squares (PLS) models as multivariate calibration were developed based on the three regions showing the specificity of TiO2 individually or in combination. For the three single peaks (636cm−1, 512cm−1, 398cm−1), least squares method (LSM) was applied to develop three univariate quantitative analysis models. One of best multivariate quantitative model having a factor of 1 gave the lowest RMSEP of 0.128, 0.129, and 0.125, respectively for prediction batches. When LSM was applied to the single peak at 636cm−1, the univariate quantitative model with an R2 of 0.9863, slope of 0.5851, and y-intercept of 0.8066 had the lowest RMSEP of 0.138, 0.144, and 0.153, respectively for prediction batches.

The in-line Raman spectroscopic method for the analysis of coating weight gain was verified by considering system suitability and parameters such as specificity, range, linearity, accuracy, and precision in accordance with ICH Q2 regarding method validation.

The proposed in-line Raman spectroscopy can be utilized as a PAT for product quality assurance as it offers real-time monitoring of quantitative changes in coating weight gain and process end-points during the film coating process.

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