Abstract
Polymer foams are difficult to characterise due to rapidly evolving physical features from liquid to porous solid. Swift changes in volume, porosity and moduli render many techniques challenging for the characterisation of the foam curing during a manufacturing process. A technique that employs the longitudinal speed of sound of an ultrasonic signal, informed by a thermokinetic model, is proposed as an in situ, in-line, non-destructive and continuous monitoring tool during the production of rigid polyurethane foams. This study demonstrates that speed of sound measurements are suitable for (a) continuous characterisation of different foaming stages in the polymer reaction and curing; (b) determining the degree of cure for the continuous monitoring of foams, and (c) predicting mechanical properties (i.e., stiffness and Poisson's ratio) of cured foam samples. The validity of this monitoring technique is confirmed by comparison with well-established methods that use physical characteristics (e.g., expansion rate, electrical properties), thermo-kinetic models and mechanical testing. This method positions itself as a monitoring tool and convenient method for determining material stiffness during production.
Highlights
Rigid polyurethane thermosetting foams are widely used as structural and insulation materials in many sectors[1] due to the broad range of physical properties attained by altering chemical formulation or manufacturing process
This paper reports the use of speed-of-sound as a method for the monitoring and characterisation of reaction stages assisted by autocatalytic thermo-kinetics that model the degree of cure in polyurethane foams
A thermokinetic-informed acoustic technique is proposed as an in-line, non-destructive method to characterise and identify reaction stages as well as a monitoring tool to follow degree of cure during bulk manufacture of a thermosetting foam
Summary
Rigid polyurethane thermosetting foams are widely used as structural and insulation materials in many sectors[1] due to the broad range of physical properties attained by altering chemical formulation or manufacturing process. The prediction of these properties presents opportunities for manufacturers who wish to fabricate engineered porous polymers tailored for hi-tech applications. When appropriately mixed, thermosetting resins (e.g., polyurethanes or epoxies) change in physical character from a viscous liquid to a gel, and to a solid through stages that have been well documented Monitoring this evolution informs the degree of cure (α) of the polymer along with properties through the development of the solid-phase moduli. A robust monitoring tool that can determine the cure state of the foam and the foam stages in situ, in-line, non-destructive and avoiding sample extraction, would add value to the manufacturing process in industrial settings
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