We recently commissioned an independent study to determine how our inks might be impacted by exposure to extreme cold temperatures either as a result of exposure in transit or in a working and storage environment outside those normally specified as optimum operational temperatures for both inks and digital printing machines alike.
Our initial concern came from anecdotal evidence from one of our suppliers who reported that he had witnessed an ink user operating in a room that was at a very low temperature (possibly around 5°C) in mid winter.
We had never considered that someone would be working in such cold temperatures inside a workspace, being used as we are to air conditioned and heated print room environments that stabilised the temperature to meet the specifications laid down by printer manufacturers that recommended operating environments of between 18c and 30c. Our concern was heightened when the distributor explained that the outside temperature where this customer was located was often as low as -12c even in the middle of the day!
Luckily for us we are located not far from the Centre for Process and Innovation (CPI) at Sedgefield in County Durham who specialize in helping SME’s not unlike ourselves and with the facilities at their disposal they were only too pleased to assist. We provided them with a brief and they set about conducting their analysis of how temperature extremes might adversely affect the performance of our inks. As a company we find it important to obtain independent evaluations to support our own analysis and on this occasion we were extremely grateful to be able to call upon CPI for such independent evaluation.
Two critical factors in ink jet printing are droplet formation at the print head nozzle and drop spreading and wetting behaviour on the substrate. The surface tension and viscosity of the ink are therefore important parameters – typical values are respectively 30-40mN/m and a few mPa.s . It was therefore agreed that CPI investigate these properties on a freshly manufactured ink and after storing the ink at elevated temperature for a week. The stored sample would also be investigated for any evidence of gross sedimentation effects. Additionally the freezing point of the ink was to be determined to check this wasn’t a factor in using the ink at low temperatures.
CPI has a range of testing equipment at their disposal and for these tests they utilized the following:
The tests were completed over a period of about 10 days and the report received indicated the following:
The ink investigated was water-based but the presence of the other elements of the ink formulation (colorants, surfactants etc.) was expected to depress the freezing point below that of water. This is indeed observed: a melting point of -12.9°C was determined by DSC on heating at 5°C/min, with significant super-cooling of around 20°C below this observed on cooling. It is therefore unlikely that freezing of the ink would occur in the range of operation temperatures discussed. The freshly manufactured sample had a surface tension nearly half that of water. This is desirable for inkjet inks and likely due to presence of surfactants that are deliberately added to the formulation: a low surface tension is desired to aid wetting of the substrate (although it should be not so low as to cause the ink to drip from the print head). The surface tension decreased by a few mN/m as temperature increased, but no dramatic changes were seen, indicating that no unexpected extra adsorption / desorption of surfactant at the ink surface was taking place.
Viscosity measurements were made at a range of shear rates between 1 s-1 and 1000s-1, which correspond to timescales of 1ms – 1s. These timescales are typically relevant for the impact, spreading and drying of inkjet inks. Viscosity measurements of the ink all showed a low viscosity (of the same magnitude as water) and the curves were relatively flat across this range. As expected, the viscosity decreased with increasing temperature but no significant changes in viscosity behaviour were seen. (A significant change could be, for example, a change in viscosity by an order of magnitude or more – as for example is seen in “shear thinning” behaviour in paints and personal care creams).
The aged sample (stored in closed container in oven at 40°C for 1 week) data for surface tension and viscosity were tested and a comparison with the freshly manufactured ink was presented. There was no significant shift in surface tension values. This implies that there was no change in the surfactant behaviour (e.g. degradation, micelle formation). The aged sample viscosities at 20°C and 5°C were slightly different from the as prepared sample but again, these were not significant differences.
Microscopy on the aged sample (taking a sub sample from the bottom of a storage tube) showed no evidence of sedimentation.
So, while not the end of the matter, with the assistance of CPI we were in a much better position to advise our distributors and customers alike on the likely outcomes that extreme environmental conditions can have on the stability or otherwise of our products.