Control Continuous Inkjet Ink Viscosity with Eastman CAB

One of the main types of inkjet printers currently used is continuous inkjet (CIJ) printers, primarily for coding and marking packaging. CIJ printers work by pumping ink from an ink reservoir, which is expelled through nozzles in the inkjet printhead. This printing process may run for up to hundreds or even thousands of hours at a time, necessitating good performance throughout. Inkjet ink viscosity is a crucial parameter that controls the print quality and smooth operation of the inkjet printer. Ink with an out-of-specification viscosity will, at the very least, reduce print quality and, at worst, cause printing operations to fail.

Solving Inkjet Viscosity Issues with Eastman Cellulose Esters 

Nitrocellulose is a common ingredient used to control viscosity in solvent-based CIJ inkjet inks for coding and marking applications. While nitrocellulose may achieve adequate viscosity control for short-term printing, its performance drastically deteriorates during the lengthy storage times that CIJ printing demands, especially under variable temperature and humidity conditions. This is likely because nitrocellulose is prone to the hydrolysis that occurs at high temperatures and humidity. 

Eastman’s cellulose esters, including cellulose acetate butyrate (CAB), offer an alternative to nitrocellulose for long-term ink viscosity control and stabilization under varying conditions. CAB has long been used in wide-format printing to provide benefits ranging from antiblocking to flow and leveling improvements to adhesion promotion. In addition to creating better long-term viscosity control, using Eastman CAB will increase the overall quality of inkjet inks. 

Cellulose Acetate Butyrate vs. Nitrocellulose Viscosity Stability

Below, the relative stability of solutions prepared with a CAB resin and a nitrocellulose (NC) resin in MEK was compared over four weeks in storage at different temperatures. As depicted, there was a significant drop in the viscosity of the NC solution at an elevated temperature, indicating that CAB provides better long-term viscosity stabilization.

Cellulose Acetate Butyrate Inkjet Ink Rheology and Stability

To investigate the impact of the rheology Eastman CAB resins provide to inkjet inks, solutions of CAB were prepared in MEK and gamma-butyrolactone, then used in ink jetting trials. The solutions were prepared with viscosities between 5 and 20 cPs and surface tensions between 30 and 45 mN/m at 25°C. These solutions were imaged near the printer's nozzles at 22°C over 5 minutes using various inkjet waveforms. Below are the results of a solution prepared with Eastman's CAB-551-0.2. Excellent inkjet ink droplets were obtained at the 07/10/11 63V waveform without ligaments attached to the nozzle, which are responsible for causing a loss of image quality.

The solution prepared with CAB-551-0.2 was also imaged to determine if there could be changes to ink viscosity stability with respect to time. There was excellent stability of the droplets at the optimized waveform at 1 minute and 5 minutes of jetting, as shown below. The lack of variation in the droplets indicates that CAB provided excellent viscosity stability over time.

Solus™ Cellulose Esters for Sustainable Inkjet Ink Formulations

Several solvent-based formulations, including inkjet inks, have recently been reformulated to be more compliant with VOC regulations that aim to reduce environmental damage. Eastman has developed Solus™ performance additives based on cellulose ester chemistry that provide many of the same rheology and performance benefits as Eastman CAB. Solus™ additives help reduce VOCs because their low molecular weight makes them compatible with high-solids ink formulations that reduce overall solvent demand.

Like the cellulose acetate butyrate solutions examined above, solutions of Solus™ 2100 were also prepared and imaged to examine ink droplet formation and viscosity stability. In the picture below, a 33% solution of Solus™ 2100 was prepared in MEK, demonstrating optimal droplet formation at a 17/10/17 63V waveform at 22°C. The solids concentration is notable in this case, as it is relatively high, reducing solvent demand and VOCs. Still, it facilitates a viscosity low enough to facilitate inkjet printing.

A 16% solution of Solus™ 2100 was also prepared in gamma-butyrolactone to gauge viscosity stability over time. The solution was imaged at 1 minute and 5 minutes during jetting, showing the same consistency and indicating that the solutions had excellent viscosity stability.

Conclusion

The quality of continuous inkjet (CIJ) printing is highly dependent on having stable inkjet ink viscosity over hundreds to thousands of printing hours. While nitrocellulose is a common resin used to control ink viscosity, it does not facilitate good long-term stability, especially when exposed to heat and humidity in storage. Alternatively, Eastman’s cellulose acetate butyrate (CAB) and Solus™ additives have been developed to provide excellent long-term stability and viscosity control that forms ink droplets with consistent quality and shape conducive to creating images with minimal defects. Solus™ additives are particularly useful for improving the sustainability of inkjet inks because they are compatible with low-VOC formulations, which reduce solvent demand through higher solids levels. Click below to discuss your ink formulations and request a sample of Eastman CAB or Solus™ today.