Date of Award

Spring 5-2019

Level of Access

Open-Access Thesis

Language

English

Degree Name

Doctor of Philosophy (PhD)

Department

Forest Resources

Advisor

Mehdi Tajvidi

Second Committee Member

Douglas W. Bousfield

Third Committee Member

Douglas J. Gardner

Additional Committee Members

Stephen Shaler

Roberto A. Lopez-Anido

Abstract

Paper coating layers are subject to various stresses and deformations in many converting processes such as calendering, printing, slitting, and folding of the paper. In some cases, products may crack during folding to generate a defect called cracking at the fold (CAF). The parameters that influence these defects are not well understood. The overall goal of this thesis is to better understand the CAF behavior as related to material properties of the coating layer.

A method was developed to produce free-standing pigmented coating layers thick enough to be tested in bending as well as tension. The mechanical properties of these coating layers were characterized for mixtures of latex and starch for two different pigment volume concentrations (PVC). Three different types of paper were coated with these formulations and their failure during printing was evaluated through a standard ink picking test. These same coating formulations were also applied on two paper grades as single and double coating layers. The cracking propensity of these samples were characterized for a range of latex and starch content. Samples of calcium carbonate marbles with binder films between them were tested in tensile tests: the results were compared with the tensile properties of the pure binder films. The failed surfaces were analyzed using Raman spectroscopy and scanning electron microscopy (SEM).

Tensile and flexural moduli of the coating layers were found to be similar and were a function of the binder content and the latex to starch ratio of the binder. However, the stress and strain at failure were higher in flexural tests compared to those in tensile tests. Papers with different basis weights and porosities were found to have different picking resistance values. The picking resistance did not correlate with the elastic modulus of the coating, but did better with the strain at failure behavior. This result indicates that the stiffness of the coating layer is not as critical during printing as compared to its flexibility.

In the folding tests, as the PVC, starch content, and paper basis weight increased, the crack area increased. Double layer coated papers with high PVC and high starch content as the top layer had more cracks in comparison with a single layer coated paper, but when the PVC of the top layer was low, cracking area decreased. Crack area decreased to a low level if the top layer was composed of a latex only system.

Increasing the starch ratio in the binder increased the tensile modulus and decreased the strain at failure. All marble rock samples failed at lower maximum stress and strain at failure values compared to pure films: this result indicates the dominance of adhesive failure. While Raman results did not indicate any residual latex or starch on the adhesively failed surfaces, SEM confirmed that at least part of the failure was still cohesive. The results indicate that if better adhesion between the pigment and the binder could be obtained, the coating layers should have improved mechanical properties and potentially better CAF resistance.

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