Process Intensification (PI) is an aspect of Chemical Engineering devoted to the design of new processes and techniques to reduce equipment size for more efficient product output. This thesis reports the first investigation directed at establishing whether ink jet printing (IJP) technology can be adapted for downscaling the direct reactive processing of polyurethanes (PU). Reaction Injection Moulding (RIM) technology represents an important step in downscaling so that a brief exploration of the RIM approach was undertaken first to identify PI features relevant to IJP technology.
As a prelude to developing the IJP processing technology, the kinetics of the chemical reactions leading to the synthesis of PUs were characterised using a range of methods including gel permeation chromatography (GPC), FTIR and Raman spectroscopy, differential scanning calorimetry (DSC) and dynamic mechanical
analysis (DMA). The impact of the initial formulation and process conditions on the microstructure of PU was established. These data served as a guide to the design of PU formulations suitable for both the RIM and IJP experiments.
The Reynolds and Weber numbers of the polypropylene glyco/isopropanol solutions proved to be unsuited to dispensing using the commercial Microdrop Inkjet Printer so that a new, IJP-type, two-stream micro-dispensing system using solenoid valves and incorporating a high-voltage electrode was designed and built. This system allowed the possibility of making two droplets collide in mid-air and to coalesce forming a spherical surface-free microreactor. This novel approach to PI produced good quality droplets of 70%w/w diisocyanate prepolymer solution in tetrahydrofurane (THF) in one stream and 60% w/w of polypropylene glycol in isopropanol in the second, albeit that droplets of the latter were some 30% larger than the former. By simultaneously triggering the reactant streams polyol droplets some 30% larger than the prepolymer droplets were formed. The resulting differences in velocities made it difficult to achieve droplet collision with simultaneous triggering of the two solenoids. However, relatively little further work will be necessary to achieve this final goal.
The reaction between two colliding droplets was simulated by adding a droplet of prepolymer to a polyol droplet in a microassay plate and monitored by FTIR (bulk measurement) and Raman spectroscopy (interfacial measurement) techniques. The
half-life for conversion was found to be between 30 and 50 ruins. A significant observation was that upon contact, a homogeneous reaction occurred at the interface leading to PU film formation. A mass and heat balance model of the PU chemistry was solved numerically. For equimolar quantities of the reactants in two drops of 100 μm diameter, it was shown that 50% conversion should be achieved in the first 2 s and 98% in approximately 60 s, which is consistent with the spectroscopic data.
Although the final goal of synthesising PU by the mid-air collision of two precursor droplets was not achieved, nevertheless, the study represents significant progress in the process intensification of PU. Key parameters have been identified and new opportunities created for further research in this direction.