Monitoring ethylene content in heterophasic copolymers by near-infrared spectroscopy; Standardisation of the calibration model

The concentration of ethylene in samples of heterophasic copolymers was monitored for 3 months by NIR spectroscopy and partial least-squares (PLS) multivariate calibration in an attempt to assess the validity over time of the calibration model. Assuming that the model would cease to be valid, we selected and monitored sufficient samples so as to initiate a process of standardisation. The samples were selected with the Kennard¯Stone algorithm and according to the representivity of their ethylene content. The monitoring techniques were univariate (Shewhart) and multivariate (T2 and Q) control plots. The results show that the system was stable, until the point at which there was a cause in variation which gave rise to an erroneous prediction of the ethylene content. This change did not vary over time. We used two techniques to standardise the calibration model: slope bias correction (SBC) and piecewise direct standardisation (PDS). They both corrected the error detected.