Effect of temperature and age on near infrared spectra of amino resins

Abstract

Quality control of amino resins must be based on reproducible and rapid methods. Fourier-transform near infrared spectroscopy (FT-NIR) has been gaining increasing interest in this context. However, it is not always possible to perform the analysis under the same conditions. Temperature and storage time, in particular, are two factors that often vary. However, their influence on the FT-NIR results is not yet well understood. This work describes how temperature and resin ageing affect the near infrared spectra of amino resins. It is shown that a previously calibrated near infrared model to assess the molar ratio of amino resins has a linear response with temperature. To counteract this effect and improve the speed of analysis, the spectral pre-processing of extended multiplicative scattering correction was used in conjunction with the loadings of water at different temperatures. This procedure was able to diminish the dependency of the model in relation to temperature for two amino resins (an R² above 95 % of a linear fit went down to below 1%). With respect to the ageing of amino resins, NIR spectra of two resins were examined for a period of 9 days. It was found that the spectra are influenced by the continuation of the condensation reactions and the formation of aggregates, which causes increase in absorbance with resin ageing. This was proven by checking the differences between NIR spectra of amino resins before and after being subjected to ultrasonic treatment to promote deagglomeration.