Dispersed particle size characterization by in-line turbidimetry during polymer extrusion

The presence of a dispersed second phase in a medium scatters light, attenuating the transmitted light intensity. This effect is maximized when the average particle size matches the light's wavelength. This work proposes an in-line optical detector which measures the attenuation of the transmitted light intensity by illuminating the medium with different monochromatic light wavelengths. Curves of normalized turbidity as a function of the dispersed particle size were simplified to a Gaussian behavior. We propose a Z parameter, function of the ratio between the transmitted light intensity measured under a particular illuminating color compared to that measured under infrared illumination, which is proportional to the particle size. The method was bench validated employing alumina particles aqueous suspensions with known particle size and during polystyrene melt extrusion traced by alumina particles and polyolefin dispersed second phase. The estimated particle sizes measured in-line during the extrusion, were in good agreement with the reference alumina particles.