세라믹 잉크의 유변학적 특성 확인하기

Optimizing Rheology

for Ceramic Inkjet Inks

 

 

 

 

Introduction


Inkjet technology and the printing flexibility it offers is enabling a rapid growth in industrial and commercial inkjet printing applications. In particular, this flexibility is driven by the inkjet printing process being non-contact i.e. the printhead does not make contact with the substrate.

As a consequence, inkjet printers can be engineered to work with a wide variety of substrates including ceramic tiles. Not only are the inkjet
printheads themselves able to be designed and configured to print on such substrates, but they can also work with a variety of different ink formulations.


Additional benefits over traditional tile printing methods such as flat screen, rotary screen and flexographic printing are improved pattern flexibility and image definition, digital image generation and storage, reduced waste and greater automation.

These benefits have driven the development and commercialisation of a number of novel printer systems since the turn of the century with a subsequent increase in the demand for compatible inks.


The inks used for ceramic tile decoration must meet certain criteria; firstly they mustbe printable and secondly they must produce the  desired decorative effect on the substrate.

The former is very much dependent on the rheological properties of the ink formulation while the latter is perhaps more dependent on the choice and size of the pigment, although the two are often interrelated.

The rheological properties of the ink base will be influenced by the pigment, particularly its concentration, while the print head may clog if the pigment size is too large, independent of its rheological behaviour.
Inkjet inks can be complex formulations consisting of the following components:


• Carrier fluid – which may be aqueous, solvent or oil-based
• Colorant – pigment or dye
• Surfactant – added to control surface tension of an ink drop
• Binder(s) – added to control the elasticity of the ink
• Dispersant – added to improve dispersion of pigment in carrier fluid


An ink formulation comprising of the above components may be either Newtonian (viscosity independent of shear rate) or non-Newtonian (viscosity dependent on shear rate).

Shear rates generated within an inkjet printhead can be extremely high as the combination of micro-scale channels in the head (typically of the order of 50μm in dimension) and the associated volume flow rates means that shear rates of %5Ccombi%20%5E%7B%205%20%7D%7B%2010%20%7D%20 to %5Ccombi%20%5E%7B%206%20%7D%7B%2010%20%7D%5Ccombi%20%5E%7B%20-1%20%7D%7B%20s%20%7D%20 or greater can be obtained.

Conversely, when the ink is being held in the supply reservoir or stored in a bottle prior to use the shear rates are relatively low, resulting
only from movement of particles under the influence of gravitational forces.

These forces are responsible for driving sedimentation, the rate of which will depend on pigment size, density and viscosity.

Most drop-on-demand ink-jet printheads require compatible inks to have a viscosity ranging from approximately 5 to 25mPas at the relevant shear rate and printing temperature. While this may be optimum for printing it is often not optimum for maintaining the pigment in suspension, in which case a high low shear viscosity would
be preferable.

This can be an inherent limitation of Newtonian inks as discussed later.
To fully characterise an ink formulation in terms of both its physical stability, and jetability, it is often necessary to measure viscosity over a range of shear rates or shear stresses, which often requires the employment of two complementary measurement techniques.

To access the low stresses and shear rates related to gravitational
processes such as sedimentation a rotational rheometer is the preferred choice as this is capable of measuring viscosities over a wide range of shear rates and shear stresses. A limitation of a rotational rheometer, however, is its inability to access very high shear rate regimes, particularly for low viscosity materials due to the onset of
flow instabilities and possible viscous heating effects at high speeds.

A more suitable technique for accessing these high shear rates (up to 106s-1) is microfluidic-based rheometry. This involves pumping a material through a narrow channel (with one channel dimension typically in the range 40-200μm) and measuring the resulting
pressure drop across the length of the channel using integrated MEMS sensors.


This application note will show how both rotational rheometry and microfluidic-based rheometry can be used to characterize and optimize ink formulations and how the size and concentration of the dispersed pigment can affect the rheological properties and ink performance.


Experimental


• Two commercial ceramic inkjet inks (labelled A and B) were evaluated using rotational rheometry, microfluidic rheometry and dynamic light scattering for size determination.


• Samples were subjected to centrifugation and the base liquid recovered for rheological measurements. Pigment volume fractions were estimated from the volume of sediment.

• Rotational rheometer measurements were made using a Kinexus pro+ rheometer with a 1°/50mm cone measuring system. An equilibrium table of shear rates was
generated for both ink dispersions and bases in the range 1-1000%5Ccombi%20%5E%7B%20-1%20%7D%7B%20s%20%7D%20.

• Microfluidic measurements were made using an m-VROCi rheometer and several flow cells, namely A05, C10 and E05, the number denoting the channel depth (05= 50μm, 10 = 100μm) and the letter denoting the pressure sensor range (A being the most sensitive MEMS sensors at 10kPa full scale, and E being the highest pressure MEMS sensors at 2MPa full scale). A 10ml syringe was used to provide the appropriate volume flow rate for the C and E chips and 1ml syringe for the A chip.

• Size measurements were performed using a Zetasizer Nano ZSP. A small amount of each ink sample was pre-dispersed in Butanone prior to measurement and transferred to a glass cuvette for measurement.


• All size and rheology measurements were performed at 25°C unless specifically stated.

 

 

  

Results and Discussion


Figure 1 shows combined data from the Kinexus and m-VROCi for ink formulations, the former providing information about low shear rate behaviour and the latter high shear rate behaviour.

Neither of these samples are particularly highly structured, however, there are clear differences between the two samples in terms of their overall viscosities.

Ink B is quite Newtonian with a viscosity of approximately 15mPas at 1%5Ccombi%20%5E%7B%20-1%20%7D%7B%20s%20%7D%20 albeit with a slight drop in viscosity as the higher shear rates are approached; the measured viscosity dropping to 13mPas at 100,000%5Ccombi%20%5E%7B%20-1%20%7D%7B%20s%20%7D%20.


Ink A is slightly more structured and shows some slight shear thinning behaviour between 1%5Ccombi%20%5E%7B%20-1%20%7D%7B%20s%20%7D%20 and 100%5Ccombi%20%5E%7B%20-1%20%7D%7B%20s%20%7D%20, which is not apparent with Ink B. At 1%5Ccombi%20%5E%7B%20-1%20%7D%7B%20s%20%7D%20 the measured viscosity is approximately 22 mPas compared with 17 mPas at 100,000 %5Ccombi%20%5E%7B%20-1%20%7D%7B%20s%20%7D%20 so as with Ink B there is a small but significant drop in viscosity as the highest shear rates are approached.





Figure 1: Flow curve showing combined viscosity measurements
for Inks A and B using the Kinexus and m-VROCi


Since viscosity is critically dependent on temperature it is important that the ink has the correct viscosity at the operating temperature of the printhead. Figure 2 shows viscosity measurements made at temperatures between 20°C and 40°C at a shear rate of 30,000%5Ccombi%20%5E%7B%20-1%20%7D%7B%20s%20%7D%20.

As one might expect there is a drop in viscosity as the temperature is
increased with both inks showing an approximate 40% drop in viscosity on going from 20°C to 40°C. What is important is that the viscosity remains within that range specified for the particular printhead being used.

Figure 2  Viscosity dependence on Temperature for Inks
                       A and B at a shear rate of 30,000s-1 using the m-VROCi

Figure 3 shows measured flow curves for the two ink bases, recovered by centrifuging the original samples. Both ink bases appear largely Newtonian with Ink A having an average viscosity of 11mPas compared with 9mPas for Ink B. The relative viscosities (ƞr) of the two inks were calculated from the ratio of the ink viscosity (low shear plateau) to that of the ink base.

This calculated value was close to 2 for Ink A and 1.6 for Ink B. This suggests that 50% of the viscosity comes from the presence of
pigment for Ink A and 40% for Ink B. The fact that the ink bases are both Newtonian also suggests that the slight shear thinning observed for Ink A Is due to the presence of the pigment and not an inherent feature of the base.

 

 

 

 

Figure 3  Measured viscosities of Ink bases A and B following centrifugation

 

 

 

The estimated volume fractions (ϕ) of pigment were 0.21 (21%) for Ink A and 0.15 (15%) for Ink B. This higher volume fraction in Ink A is consistent with the slight shear thining observed in this formulation.

Note that for inkjet inks with higher particle loadings than those tested in this application note, the degree of non-Newtonian behavior is likely to be much more significant, and as such emphasises the importance
of flow curve measurements up to very high shear rates to ensure ink compatibility with the printhead under ‘in use’ conditions.


Using the volume fraction and the pre-determined ink base visc


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