A method of measuring the extent of cure of a binder in a fibrous product is provided. The method comprises (a) providing a sample of a fibrous product, the fibrous product comprising fibers and a binder, (b) subjecting the sample to infrared spectroscopy to generate a spectral analysis, and (c) determining a binder cure ratio of the binder in the sample using the spectral analysis. The binder has carboxylic acid groups and alcohol groups that crosslink to form ester groups as the binder cures, and the binder cure ratio comprises a ratio between infrared absorption corresponding to a bond of the carboxylic acid groups in the binder and infrared absorption corresponding to a bond of the ester groups in the binder.