Polymer degradation during continuous ink-jet printing

Results are reported for the first time on the possibility and cause of polymer degradation in a full commercial inkjet printer. The behaviour of three linear poly(methyl methacrylate) (PMMA) samples, having Mw 90, 310 and 468 kDa respectively, in a continuous inkjet (CIJ) Domino A-Series+ printer is investigated and compared with earlier reported results for two experimental drop on demand (DOD) printers, a Dimatix DMP-2800 and a Microfab single nozzle glass capillary. Despite all three printheads having equivalent strain rate at the nozzle tip (ε) no degradation is observed within the CIJ head alone whereas single pass degradation is observed in both DOD printheads. This can be attributed to a consequence of a number of factors including the slightly greater nozzle diameter, and different nozzle geometry, which includes cone angle and nozzle cylinder length, for CIJ, the different mode of drop generation or the higher polymer concentration investigated. It should also be noted that the flow through the CIJ nozzle is continuous, where it is pulsed in the case of DOD. The calculated maximum strain rates are similar, but in DOD it is being periodically ramped up and switched off. The main source of polymer degradation in CIJ printing through a CIJ Domino A-Series+ printer is found to be the continuous recycling of ink through the pump resulting in mechano-chemical polymer degradation. Molecular weight degradation proceeds by random scission, and is obtained at the lower shear rates experience as a consequence of the longer residence time due to mechano-chemical damage in the pump. These results have significance in the total design of inkjet systems for the delivery of high molecular weight polymers and materials sensitive to mechano-chemical degradation and highlight the need for careful consideration when moving from laboratory based print tools to full scale application. © 2014 The Authors. Published by Elsevier Ltd.