Hyperspectral near Infrared Image Analysis of a Phenol Formaldehyde Resin Curing Reaction

Abstract

In the production of sandpaper, a phenol formaldehyde polymer bonds the abrasive material to a backing material made of paper or a textile. The reaction takes place at elevated temperatures (90–120°C) for a long time (at least eight hours) and is thereby a very energy-demanding process step. A possible future application would be to use near infrared (NIR) hyperspectral imaging (1000–2498 nm) for (1) monitoring the curing reaction and determining an end point for it and (2) for identifying in homogeneous regions with low adhesion in the final product. A feasibility study was carried out on four series of resin-coated backing materials without abrasives. These were imaged at half hour intervals for eight hours of curing using a NIR line scan imager. In order to analyse the influence of the backing material on the net NIR signal from resin-coated samples, spectra of the backing material (paper, textile) were also collected using a moving grating NIR spectrometer (1100–2498 nm). Results from principal components analysis of the hyperspectral images indicated that the reaction was stabilised after five to six hours, although it continued slowly for at least 16 more hours. A relevant question was when to finish the heating (curing) and still obtain a final product of high quality. Partial least squares regression models for predicting the curing time were thus also evaluated. A calibration made on image mean spectra was used for predicting the curing time of each pixel in the full set of hyperspectral images. The predicted images showed the curing progress, in homogeneous regions where the reaction had progressed to a slower extent and other physical abnormalities (for example air bubbles). Pixel prediction distribution analysis of the images was found useful for determining the significant number of components of the proposed regression models.