Date of Award

Spring 5-11-2019

Level of Access

Open-Access Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical Engineering

Advisor

Douglas Bousfield

Second Committee Member

Hemant Pendse

Third Committee Member

Albert Co

Additional Committee Members

Douglas Gardner

Mehdi Tajvidi

Abstract

The mechanical properties of pigmented coatings are important for a number of situations; including coated paper, architectural paints, and structures in flexible lithium ion batteries. Coated paper and board undergo a variety of post coating application processes which have the potential to cause serious quality problems such as cracking, picking, and crack-at-the fold (CAF). Because a large number of parameters are known to influence the results, fundamental models are needed to help describe these processes and link them to the coating formulations and to the defects.

A discrete element method (DEM) computer model was developed to describe the pigment level deformation of the coating layer. The model is based on calculating the forces between particles as they move relative to each other and undergo tension or compression. For the case of tension, a non-linear stress-strain relationship was developed that is similar to the behavior seen for pure binder films – data for the pure binder are inputs into the model. In the case of compression, a repulsive force is used that is linear with strain. This thesis is the first time that a DEM was used to model bending, to include the influence of starch, and to model two coating layers. The model was compared to recent experimental results in the literature for free-standing coating films using different ratios of pigment to binder and also various combinations of latex and starch in the binder systems.

The two dimensional version of the model was set up using uniform spherical particles to represent the paper coating pigments. For both tension and three-point bending, the model was able to predict cracking in accordance with the experimental data. The model’s results followed the same trends and were of the same order of magnitude as the lab data. However, differences between the two sets of data did exist, which could be attributed to such causes as issues when making the coating films in the lab, starch impacting the packing, assuming only cohesive failure, the use of spherical particles, and the assumptions made for the simulated packing. The two-dimensional model also was used to simulate the printing event via an out-of-plane tension event and by applying a moving force boundary condition. Picking correlated to both the experiments and the models for the strain-at-failure (STF) and not for the elastic modulus or for the ultimate stress. The two-dimensional model also was applied to two layer coatings. The model agreed with the literature in that the starch-rich layers of high coat weight were more prone to cracking. Furthermore, the two-layer model agreed with pilot and mill results by predicting less cracking with a thick, flexible bottom layer and a thin, stiff top layer.

The three-dimensional model using the packing distribution of uniform spheres, of bimodal size distributions, and of full particle size distributions improved the predictions relative to the two-dimension cases. The results with uniform spheres showed the modulus, maximum stress, and strain-at-failure to be well predicted except for the maximum stress being underpredicted for cases near the critical pigment volume concentration (CPVC). In addition, the strain-at-failure tended to be overpredicted. When the model used the bimodal and full distributions for packing, the predictions improved. The model overpredicted the modulus and underpredicted the maximum stress, but the predictions were close in some cases, especially when using the full distribution. In addition, the STF showed good agreement between the predictions and the lab data when starch was part of the binder system. Discrepancies still exist between the model predictions and the experimental data, and these differences can be attributed to many factors including the method of packing. The model showed the modulus and the maximum stress to increase directly with the packing density. These results are in accord with the expectation that a tighter initial packing leads to higher local strains, which lead to increased modulus and stress.

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