Method for in-line analysis of a composite product in a machine for the production of absorbent sanitary articles

A method for in-line analysis of a composite product, the method comprising:

• a preliminary step and an operating step,
• wherein said preliminary step comprises:
• arranging two samples of target materials, which are part of said composite product, composed of cellulose fluff and granular superabsorbent polymer (SAP), respectively,
• carrying out a spectral analysis of said two samples of target materials,
• determining spectral reflectance or transmittance data of each of said two samples of target materials, and
• identifying a reference wavelength at which a difference between the spectral reflectances or transmittances of said two samples of target materials is maximum;
• wherein said reference wavelength is between 1300 nm and 1500 nm;
• and wherein said operating step comprises:
• advancing said composite product in a feed direction in a machine for producing absorbent sanitary products, at a speed from 100 to 800 m/min,
• illuminating said composite product with electromagnetic radiation having an emission range including said reference wavelength,
• receiving electromagnetic radiation reflected by, or transmitted through, said composite product by an optical sensor sensitive to said reference wavelength, and
• processing signals provided by said optical sensor, and obtaining information on characteristic parameters of said two samples of target materials contained in the composite product.