Read bellow the latest analysis of printed ink by Dr. Andy Shipway.
Microscopic Analysis of Printed Ink
Dr. Andy Shipway
The microscopic analysis of Dip-Tech’s inks shows a homogeneous and bubble-free structure that is fused to the substrate glass to provide a continuous structure. Fired ink thickness is increased by 1.4 microns for each “100%” of printing opacity.
Dip-Tech produces ceramic inks for glass substrates that are cured by firing at temperatures of 600-670°C. The firing process converts the powdery dry ink into a hard and durable layer that can withstand tough mechanical and chemical conditions for a period of many years.
This report details a microscopic analysis of the Dip-Tech inks, in particular their fusion to the glass during the firing process, in order to help understand the ink properties as well as to more rigorously characterize the ink and detail some of its differences from other glass printing technologies.
Dry, unfired ink structure
The dried, unfired ink layer consists of glass frit, pigment, and organic additives. The organic materials act as binders to prevent the frit and pigment layer from disintegrating during handling. Electron micrographs of this structure are shown below:
Scanning electron micrographs of a cross-section (fracture surface) through a printed ink layer (400% printing opacity), after drying but prior to firing. Left: 5000x magnification, with the glass fracture surface at the upper left. Right: 50,000x magnification, showing frit and pigment particles
The thickness of this layer is measured at approximately 12.5 microns, significantly (as expected) less than the wet ink thickness of ~40 microns. It is clearly seen that the frit particles (the larger particles) and the pigment particles (the smaller particles) are intimately mixed. This is important to note, since any separation that had taken place during drying could result in poor properties such as strength, gloss, and opacity.
Fired ink structure
During firing, the glass frit particles melt, coalesce, and fuse with the glass surface while encapsulating the pigment particles. Organic materials are burned off and the grainy structure of the unfired layer collapses to a much denser and harder structure. Electron micrographs of a fired ink layer are shown below:
Scanning electron micrographs of a cross-section (fracture surface) through a printed ink layer (400% printing opacity), after firing. Left: 5000x magnification, with the glass fracture surface at the lower left. Right: 20,000x magnification, showing embedded pigment particles
These samples are prepared by fracturing a fired printed glass and imaging the fracture surface. It is striking to see that the fracture surface formed when the glass breaks is continuous through both the glass substrate and the ink layer. This observation demonstrates that the ink is fused to the glass at a molecular level, forming essentially a single continuous piece of glass (even though the precise elemental makeup of the different regions is not the same).
The fired ink layer has an even thickness and a fat surface. It is also seen from the closer view that the pigment particles are well separated and homogeneously distributed. All these characteristics lead to a high-quality end product.
Fired ink layer thickness
The final thickness of the fired ink layer is dependent on the printing opacity (i.e. the printed ink thickness). Firing produces a dense structure, around half the thickness of the dried, unfired ink and one sixth of the thickness of the wet ink layer. The graph below outlines typical fired ink layer thickness and how it related to printing opacity:
Plot showing the measured thickness of an ink layer against the printed ink thickness. Measurements were made by electron microscopic analysis of fracture surfaces
As the plot clearly shows, each additional “100%” of printing opacity results in an increase of fired ink thickness of ~1.4 microns. However, a remaining 0.8 microns of ink thickness remains unaccounted for. This may result from the blurring of the boundary between fused layers, or from another systematic measurement phenomenon.
Fusion interface layer
During the firing of the ink onto the glass substrate, the frit in the ink melts, and the substrate exceeds its glass transition temperature (Tg). This results in a condition where the two materials can fuse together and even mix at the molecular level to some extent, with metal ions diffusing between the two layers. Consequently, and unlike in polymer-based inks, there is no formal defined interface between the substrate and the ink, and the substrate-ink monolith behaves in many ways as a single material. Exemplary micrographs are shown below:
Scanning electron micrographs of cross-sections (fracture surface) through printed ink layers. Left: 20,000x magnification, fired Dip-Tech ink, with the glass fracture surface at the upper right. Right: 1,000x magnification, polymer-based ink, with the glass fracture surface at the lower right.
In the case of the Dip-Tech ceramic ink, as discussed above, there is a clean and continuous fracture surface going through the substrate and ink layer. In contrast, a polymer ink layer can be seen to behave distinctly from its substrate – a property that ultimately can result in cracking, peeling, or other undesirable behavior.
Closely scrutinizing the interface between the ink and substrate layers, it can be seen that a sharp boundary is seen only in the polymer ink. Although the mixing between substrate and frit only occurs on the nanometer-scale, some indistinctness can be seen in the micrograph. However, the diffusion of ions between the two layers can be seen more convincingly by the use of EDAX to analyze the elemental composition of the sample at various points. This analysis picks out bismuth ions inside the substrate glass layer, which must have originated from the frit used in the ink.
The firing of the ink results in the collapse of a structure containing a certain amount of voids and organic materials. Consequently, a significant amount of gasses must be expelled from the structure during firing. Any remaining trapped gas will result in a bubble, which can alter the optical properties of the ink and provide a weak spot. To some extent, the minimizing of bubbles can be achieved by optimizing drying and firing conditions. An example of trapped bubbles is shown below:
Scanning electron micrographs of a cross-section (fracture surface) through a printed ink layer, showing bubbles of trapped gas.
Dip-Tech’s inks in general fire to produce very few bubbles, and pictures such as that above are made only by a careful search.