Influence of hydroxyl content of binders on rheological properties of cerium–gadolinium oxide (CGO) screen printing inks

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Abstract

The influence of hydroxyl content of binders on rheological properties of screen printing inks is investigated. The actual amount of hydroxyl groups is correlated to the level of hyper-entanglement that characterizes the binders in solution. Three of the most used binders (ethyl cellulose, and two vinyl resins) were selected and characterized in solution via viscosimetry method. A high degree of hyper-entanglement was observed for ethyl cellulose polymers, whereas a mitigated effect characterized the two vinyl resins. Cerium–gadolinium oxides (CGO)-based inks, prepared using the selected binders, were investigated by means of rheology. The vinyl resin at higher hydroxyl content and low level of hyper-entanglement was demonstrated to impart superior printability properties.

Introduction

Screen printing is a simple and low-cost manufacturing technique extensively used for the fabrication of ceramic films with a thickness in the range of 10–100 μm.1 It is a flexible process also compatible with high-throughput production methods as roll-to-roll and sheet-to-sheet.2 These traits make the screen printing attractive for a large variety of applications, including energy related technologies such solid oxide fuel cells,3 and solar cells.4 The process mainly consists of three steps5: (i) the application of a tangential stress on the ink, (ii) the passage of the ink through the screen mesh, and (iii) the recovery of the ink structure once the tangential stress has been removed and the ink has been deposited on the substrate. In such a process, the rheology of inks plays a central role for producing high quality films.5, 6 An ideal ink should exhibit shear thinning behaviour during the first step, with a sufficiently low viscosity at the second step to allow flow through the mesh, and a thixotropic character during the structure recovery step.6 Such requirements can be accomplished by optimizing the formulation of inks.2, 6, 7, 8, 9, 10 For correct design of an ink, a primary requirement to be fulfilled is the preparation of stable and well dispersed colloidal suspensions.11 Traditionally a high ceramic loading is required to promote fabrication of dense and defect-free structure in the sintered product.11 However, the influence of the ceramic loading on the printability and levelling properties of the inks,9 green density of the deposited films12 and their shrinkage13 during the sintering is still a debating discussion. For instance, recently Mücke et al. have demonstrated that as long as the ceramic suspension is stable and well-dispersed, no impact of the ceramic loading on the film green density is observed.12 A further important aspect is the selection of an adequate binder to promote the formation of a stable gel structure in the ink, required to prevent the sedimentation of ceramic particles and to impart the proper degree of viscoelasticity and thixotropic character. The inner network is generated through the development of hydrogen bonding between binder hydroxyl groups and surface oxygen of ceramic component.7 Therefore, binders containing hydroxyl functions are usually used e.g. ethyl cellulose in moderately polar to low polarity solvents.7 The strength and stability of the gel structure is largely determined by the content of the hydroxyl functions. However, it is worth mentioning that the effective concentration of hydroxyl function can diverge from the nominal one, as a consequence of intra-molecular interactions among the polymer chain sub-units, referred to as hyper-entanglement.14, 15, 16, 17 This phenomenon has been found to occur for polysaccharide at concentrations which are comparable with the typical binder concentrations in inks (1–2 wt%). A number of authors have conducted investigations to clarify the origin of this association.14, 15, 16, 17 Actually, all these studies converge in indicating a possible association among hydroxyl functions (via H-bonding).14, 17 This might impact all applications for which the formation of H-bonding network is required, including ceramic screen printing inks. The most widely used method for observing and assessing the level of hyper-entanglement is the viscosimetry method, which allows a quick and inexpensive determination of relevant polymer characteristics. The parameter of most interest is the intrinsic viscosity (hereafter denoted [η]) that describes the inherent ability of isolated polymer molecules (generally any solute in solution) to increase the viscosity of the solvent.18, 21 It depends on the polymer molar mass, thus when polymers at different molecular weights are compared, the use of the coil overlap parameter (c*[η]) is preferred. This further parameter is independent on the molar mass, thus leading to the master curve22 which enables a direct comparison among different polymers. It is a measure of the (hydrodynamic) volume occupied by the polymer coil in the solution.22

The main motivation for this work was to correlate the level of hyper-entanglement of three classes of polymers, estimated using viscosimetry based methods, to the rheological properties of inks prepared using the mentioned polymers as binders. Ethyl cellulose (EC), polyvinyl butyral (PVB) and polyvinyl acetal (PVAcetal), all containing hydroxyl functions and widely used as binders,23 were characterized in a solution of a glycol ether. For all types of polymers, a range of molecular weights were explored. [η] was estimated and used for assessment of the degree of hyper-entanglement. The results served as benchmarks for the design of ink formulations. In the following step, a highly hyper-entangled ethyl cellulose polymer and two vinyl resins (with different content of hydrogen bonding functionalities, i.e. OH-groups) were selected and further investigated as binders for gadolinium doped ceria (CGO)-glycol ether based inks. The developed inks were characterized by rheological methods with specific attention given to gel structure stability, yield properties, shear thinning behaviour, and thixotropic character.

Section snippets

Materials

Commercial gadolinium doped ceria powder (Gd0.10Ce0.9O2), (Fuel Cells Materials, 99% purity) was used as received. The powder specific surface area (SSA) (6.01 m2/g) was determined by BET (Brunauer–Emmett–Teller; Austosorb 1-MP, Quantachrome Instruments, Boynton Beach, Fl, USA) isotherm measurements. The particle size distribution was determined using a multi-wave length LS13 320 laser diffraction particle size analyzer (Beckman Coulter, Fullerton, USA). CGO phase and purity were characterized

Results and discussion

For clarity, this section is divided into two sub-sections. The first sub-section refers to the characterization of the binders in solution of glycol ether. In the second sub-section the rheological properties of the developed inks are discussed and compared in terms of the different level of hyper-entanglement.

Conclusions

Correlation between the amount of hydroxyl functions for polymers and their ability to impart the desired rheological properties to CGO-glycol ether based inks was established. The required affinity between the polymers selected (ethyl cellulose and the two vinyl resins) and the glycol ether solvent was demonstrated and hyper-entanglement phenomenon observed. The effect of the amount of hydroxyl groups on properties of CGO-based inks containing only binder as organic additive was investigated.

Acknowledgements

Financial support from Danish Energy Agency (EUDP-Energy Technology Development and Demonstration Programme) through the projects Fuel cells put to work (project ENS-64010-0052) and SOFC Accelerated-Development to Accelerate Field Demonstrations (project 64012-0225) is gratefully acknowledged. The authors acknowledge Lene Knudsen for valuable assistance in the experimental work.

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