Spectroscopic diagnostics of morphological changes arising in thermal processing of polypropylene

Abstract

Polypropylene is considered as a representative thermoplastic matrix for advanced composite materials that have some advantages in various engineering applications. Wide-range infrared optical properties of polypropylene are important for combined heat transfer modeling in these composite materials, which are semitransparent in a considerable part of the whole spectral range. This study is focused on optical properties of polypropylene in the visible and near-infrared ranges because the measurements in these ranges exhibit a stronger effect of the processing temperature used in the material manufacturing. The experimental study is based on spectral measurements of both the normal-hemispherical reflectance and transmittance of polypropylene samples. The main characteristics of volumetric absorption and scattering are identified using the inverse problem solution based on the modified two-flux approximation, which is sufficiently accurate to determine the hemispherical characteristics of the radiation field in the range of the problem parameters. In particular, the effect of a relatively strong scattering is observed at the absorption peaks in the near-infrared range. An approximate theoretical model based on spectroscopic data is developed to estimate morphological changes arising in thermal processing of polypropylene at different temperatures.

The role of microcrystalline structure on optical scattering characteristics of semi-crystalline thermoplastics and the accuracy of numerical input for IR-heating modeling

“…4 (b) that shows the T λ-directional values of the samples with different X c (%). The same trend from both the analyses are in good agreement with [12,17] where it was concluded that the size of a scatterer in PP semicrystalline polymer is probably too small. Therefore the optical scattering becomes negligible in MIR range due to the increase in wavelength as the size of the scatterers in PP get considerably smaller than the applied wavelength.…”

“…The PE polymer in the form of pellets was injection molded and the plate-like PE samples with dimensions of 78 × 78 × 2.2 mm were fabricated. The main reason to fabricate samples via injection molding is to obtain void free samples as the void formations may affect the optical properties of polymers as reported in [12,17]. The injected plate-like samples were then cut into small pieces and compressed in order to obtain thin samples with controlled thickness.…”

“…It should also be noted that the T λ-directional measurements on the different samples, that were prepared with identical thickness and thermal treatment (slow or fast cooled), showed that T λ-directional may vary around 3-5% in NIR range whereas such a change was not observed for the T λ-hemispherical measurements. This is probably induced by the optical scattering -microcrystalline structure relation which may change by the orientation [12], or position of the sample at each spectroscopic measurement. In Fig.…”

“…Basically, the term -optically heterogeneous medium-represents the discontinuities existing inside of a heated medium that causes to change the direction of propagated radiation, named optical scattering. These discontinuities -also called scatterers-may be fiber-matrix interface in thermoplastic composites [8,9], filled particles [10,11] or microcrystalline structure of semicrystalline polymers [10,12]. Considering optically heterogeneity of a medium, some numerical models were proposed for simulating laser transmission welding (LTW) of filled thermoplastics [11,13] and laser-assisted tape placement (ATP) processes of thermoplastic composites [14].…”

Near-infrared optical properties of a porous alumina ceramics produced by hydrothermal oxidation of aluminum

“…On the contrary, appearance of strongly absorbing regions comparable in size with the wavelength may lead to an additional scattering. It was demonstrated in experimental and theoretical studies of infrared properties of a nanoporous silica material [45] and also morphological changes arising in optical properties of polypropylene after thermal treatment [46] that there are some cases when scattering depends on absorption.…”