1. The Dispersion Stability of Upconverting Nanoparticle Inks
Khadijah Cessac (Southern University and A&M College)
Drs. Jon Kellar, William Cross, Jeevan Meruga
Abstract
Upconverting Nanoparticles (UCNP) in ink formulation are on the rise in the realm of security printing and anti-counterfeiting. Being knowledgeable about the ink’s stability and behavior under certain conditions is important because it allows scientists and companies to continuously learn key aspects to bettering ink formulations. In order for these inks to be used commercially in security printing certain boundaries and expectations must be met. For example, most companies lean towards water based inks because they are easy to produce, are environmentally friendly, and most importantly, are cost effective. While the advantages of upconverting inks are quite beneficial, there is little literature on stability testing of these inks. Therefore, the purpose of this research was to first formulate a water based ink that had a surface tension and suitable viscosity for printing purposes and then create a method to determine the stability of these UCNP inks.
Each day, four values were recorded:
Mass (g) empty glass slide
Mass (g) of 200 µL Sample on the slide
Mass (g) of slide after being heated to 300°C (573K)
Mass (g) of slide after being heated to 550°C (823K)
Additional surface tension and viscosity testing was also conducted
Figures 2-5 show the rheology and stability results obtained
Stability Testing Results
Introduction
The counterfeiting industry has had continuous success over the years (counterfeit items have been produced in a broad spectrum including pharmaceutics, electronics and critical documents, etc.) In 2015 the “Projected Value of Global Trade in Counterfeit and Pirated Goods” was $1.77 Trillion. To counteract this rising issue, several measures have been taken to improve security printing and anti-counterfeiting applications.
Two specific aspects important to security printing:
formulation and stability of ink (the ability of dispersion to resist coagulation – more specific to UCNP ink formulation)
inks ability to print efficiently
In standard ink-jet printers: two dimensionless numbers important in understanding the formation and behavior of liquid drops:
Reynolds (Re) number reflects the relationship between inertia and viscosity.
Ohnesorge (Oh) number reflects the relationship between viscosity and the combined inertia and surface tension.
In addition to the printability, the ink the formulation to be used will produce a covert and high-quality security feature due to the presence of β-NaYF4 nanoparticles.
Figure 2. Ambient samples on Glass Slide After Heating
Viscosity & Surface Tension Results
Figure 3. Viscosity Measurements of 0.3 wt% & 0.6 wt% UCNP Inks (Left)
Surface Tension Measurements of 0.3 wt% & 0.6 wt% UCNP Inks (right)
Figure 5. Weight % After Heating to 573K and 823 K
Cold 0.6 wt% sample (top); Hot 0.6 wt% sample (bottom)
Stability Testing Results
Conclusion
Stability Testing: Mass
The mass remaining after heating to 823K varies from day-to-day but we do not experience a significant loss of nanoparticles.
Surface Tension
(0.3 wt%): Average Surface Tension = 40.6 mN/m
(0.6 wt%): Average Surface Tension = 41.9 mN/m
Viscosity
(0.3 wt%): Average Viscosity = 1.28 cP
(0.6 wt%): Average Viscosity = 1.16 cP
Parameters for thermal inkjet printer (HP TIPS)
surface tension: mN/m
Viscosity: cP
The inks in this research coincide with the ink jet parameters thus making them suitable for printing.
Figure 1. Schematic diagram showing the operating regime for stable operation of drop-on-demand inkjet printing.
Objective
The research objective is to determine the stability of upconverted nanoparticle ink to gain an understanding of what is the optimal conditions for this ink and to approximate the shelf-life of upconverted nanoparticle ink.
Experimental
Ink formulation – a water-based ink was created through an emulsion process. The ending base formulation resulted in 85% water, 15% glycerol, and 0.2wt% sodium dodecyl sulfate (SDS). Initial surface tension and viscosity testing was performed on this ink formulation to ensure suitable parameters for printing purposes.
Stability Testing – samples of containing 0.3 wt% and 0.6 wt% β-NaYF4 nanoparticles were held in three different storage conditions:
Cold (≈4°C)
Hot (60°C)
Ambient (≈25-30°C)
Acknowledgements
This work was made possible by the National Science Foundation REU Security Printing and Anti-Counterfeiting Site EEC
Additionally, I would like to thank Dr. Grant Crawford and the SPACT program for granting me this opportunity. Thanks to Dr. Alfred Boysen for his contribution in technical writing. Thanks to Fidel D. Ruz-Nuglo for assisting in the TGA Test. Thanks to Drs. Stanley May and Aravind Baride for their collaboration and input throughout this process.
Figure 4. Weight % After Heating to 573K and 823 K
Cold 0.3 wt% sample (top); Hot 0.3 wt% sample (bottom)