a) providing two or more calibration samples of conventional flexible polyurethane foams, two or more calibration samples of high resilience (HR) flexible polyurethane foams, and two or more calibration samples of viscoelastic flexible polyurethane foams, and obtaining a mid-infrared (MIR) spectrum of each calibration sample, wherein the MIR is defined by a whole spectral region from 4000-400 cm⁻¹;

b) carrying out a spectral pre-processing of the spectra of all the calibration samples, and then a first principal component analysis (PCA) in the spectral region from 1800 to 1425 cm⁻¹ to define a first library that allows distinguishing viscoelastic polyurethane foams;

c) carrying out a spectral pre-processing of the infrared spectra of conventional and HR polyurethane foam calibration samples, and then a second PCA in the spectral regions from 1775 to 1625 cm⁻¹ and from 970 to 725 cm⁻¹ to define a second library that allows distinguishing conventional from HR polyurethane foams with one principal component;

d) obtaining the MIR spectrum of a sample of polyurethane foam and, based on the first and second libraries, classifying the sample of polyurethane foam as a conventional, a HR or a viscoelastic polyurethane foam, or as a foam that is neither a conventional, a HR or a viscoelastic polyurethane foam.