Near-Infrared Spectroscopic Evaluation of the Water Content of Molded Polylactide under the Effect of Crystallization

Abstract

During melt processing, the moisture inside polylactide (PLA) easily induces hydrolysis, which deteriorates the mechanical and thermal properties of the product. The state of dryness of resin pellets must be monitored to prevent PLA hydrolysis. In this study, near-infrared (NIR) spectroscopy was applied to measure water content in PLA. In addition, the shape of the NIR spectrum is also affected by crystallization, which could lead to a reduction in the accuracy of evaluating the water content. The objective of this research is to construct a robust model for estimating the water content with varying dispersive extents of crystallization. Two methods for estimating water content measured during a drying process were conducted: the integration of absorbance and partial least squares (PLS) regression were conducted to estimate the water contents in PLA considering the effect of crystallization. The slope of the calibration line of the water content obtained from integrating absorbance varied between PLA with different crystallinities. This is due to the overlap between the NIR band of water and that of PLA crystal in the range of 5100-5400 cm. We found that the shape of the NIR spectrum was changed by crystallization, and the crystallinity, compared to the thickness of lamellae, was the dominant factor determining such a change of NIR spectra. The PLS model of water content constructed from only amorphous PLA showed large error of estimation in crystallized PLA. In contrast, the PLS model constructed from both amorphous and crystallized PLA estimated the water contents with lower errors. This was because latent variables obtained from both amorphous and crystallized PLA cancelled the effect of crystallization on NIR spectra.

Application of polarized IR / NIR spectroscopic imaging for analysis of polymer products

“…By calculating the chemical composition, the reaction ratio, crystallinity etc. from each spectrum, we can obtain the chemical image of the sample [1].…”

Near-Infrared Spectroscopic Method for Monitoring Water Content in Epoxy Resins and Fiber-Reinforced Composites

“…Therefore, an alternative technique for measuring water content in epoxy resins and fiber-reinforced composites is required. NIR spectroscopy, e.g., FT-NIR, is a promising technique for water content monitoring [ 21 , 22 , 23 ]. Due to the high transmission of NIR light, its applicability to products with sizes in the mm range is usually possible [ 21 ].…”

“…Recent improvements of NIR spectrometers, resulting in a dramatic increase in their performance, e.g., acceleration of measuring time, have further lowered the threshold for using this method for process monitoring [ 21 , 22 ]. NIR has a broad range of practical applications, both in the laboratory and industry, the evaluation of diffusant content being one of those.…”

“…NIR has a broad range of practical applications, both in the laboratory and industry, the evaluation of diffusant content being one of those. For instance, NIR spectroscopy has long been used in such fields as medicine, food and polymers [ 21 , 26 , 27 , 28 ].…”

Visualization of hydrolysis in polylactide using near‐infrared hyperspectral imaging and chemometrics

“…Furthermore, the high transmittance of NIR light enables nondestructive measurements of relatively thick products, even those in the millimeter range. Several studies have been reported on using NIR to evaluate phase separation, curing reactions, practical polymer processing, moisture, and crystalline structure . Therefore, NIR spectroscopy is a suitable method for the online evaluation of PLA products.…”