Abstract
A visualization system to observe crystal and bubble formation in polymers under high temperature and pressure has been developed. Using this system, polymer can be subjected to a programmable thermal treatment to simulate the process in high pressure differential scanning calorimetry (HPDSC). With a high-temperature/high-pressure view-cell unit, this system enables in situ observation of crystal formation in semi-crystalline polymers to complement thermal analyses with HPDSC. The high-speed recording capability of the camera not only allows detailed recording of crystal formation, it also enables in situ capture of plastic foaming processes with a high temporal resolution. To demonstrate the system’s capability, crystal formation and foaming processes of polypropylene/carbon dioxide systems were examined. It was observed that crystals nucleated and grew into spherulites, and they grew at faster rates as temperature decreased. This observation agrees with the crystallinity measurement obtained with the HPDSC. Cell nucleation first occurred at crystals’ boundaries due to CO2 exclusion from crystal growth fronts. Subsequently, cells were nucleated around the existing ones due to tensile stresses generated in the constrained amorphous regions between networks of crystals.
Highlights
This article presents an expanded research study based a conference paper published in the Society of Plastics Engineers Annual Technical Conference 2012 [1].High-pressure differential scanning calorimetry (HPDSC) is a widely used technology to determine the melting and crystallization kinetics of semi-crystalline polymers in the presence of dissolved gas at a high temperature and pressure [2,3,4,5]
This paper presents the development of a static foaming visualization system with a high-speed camera and active heating and cooling controls
The accurate heating/cooling control is essential to studies where the thermal history of a plastic sample impact its foaming behavior, notably the foaming of semi-crystalline polymers at temperatures where crystallization occurs, such as in many bead foaming processes
Summary
This article presents an expanded research study based a conference paper published in the Society of Plastics Engineers Annual Technical Conference 2012 [1].High-pressure differential scanning calorimetry (HPDSC) is a widely used technology to determine the melting and crystallization kinetics of semi-crystalline polymers in the presence of dissolved gas at a high temperature and pressure [2,3,4,5]. Many HPDSC systems have limited operating pressure (e.g., 150 °C) is used. This thermal analysis could not provide direct information on the crystal population density and sizes. In this context, Mezghani and Philips [6] have incorporated a miniature high-temperature/high-pressure view-cell onto a polarized optical microscope (POM) to observe crystal formation of semi-crystalline polymers under a high temperature (200 °C) and pressure (200 MPa). The scientific knowledge of crystallization and melting behaviors generated from the thermal analysis and optical microscopy is imperative to the advancement of polymer science
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