Measurement of moisture in PVB

PVB (Polyvinyl butyral) is used as an interlayer in car windscreens. The moisture content of the foil is an important property for the windscreen’s ability to withstand foreign object impacts without breaking. During production the PVB foil undergoes several processing steps. Finally it is cut into pieces that fit the size of the windscreen.

An in-line version of the omt RT (reflection/transmission) measurement system is used to control this process.

Water and PVB molecules absorb light in the infrared region of the optical spectrum. This is detected with an omt ctf-f60 IR spectrometer. It features a cooled InGaAs sensor and is optimized for the wavelength range from 1100 nm - 2200 nm.

The graph shows the measured spectrum, which takes less than 500 milliseconds to accumulate. The CH₂ content, which is proportional to the foil thickness, can be calculated from the peak around 1700 nm and the H₂O content from the peak around 1950 nm. Finally the moisture is calculated by relating foil thickness to water content.